[man pages]: Define page-local `MR` fallback.

[man pages]: Provide page-local fallback definition of new `MR` macro.
I didn't think I was going to have to do this, but the premier site on
the web for viewing Linux man pages, Michael Kerrisk's man7.org, has
been pulling snapshots of the pages themselves without upgrading the
underlying macros, and so man page cross references set with `MR` are
getting lost in its presentations.  (I acknowledge: Ingo Schwarze warned
me something like this could happen.)  This definition is intended as a
stopgap measure only.  I want to revert this after groff 1.23 is
released and has spread to some reasonable degree.

* contrib/chem/chem.1.man:
* contrib/eqn2graph/eqn2graph.1.man:
* contrib/gdiffmk/gdiffmk.1.man:
* contrib/glilypond/glilypond.1.man:
* contrib/gperl/gperl.1.man:
* contrib/gpinyin/gpinyin.1.man:
* contrib/grap2graph/grap2graph.1.man:
* contrib/hdtbl/groff_hdtbl.7.man:
* contrib/mm/groff_mm.7.man:
* contrib/mm/groff_mmse.7.man:
* contrib/mm/mmroff.1.man:
* contrib/mom/groff_mom.7.man:
* contrib/pdfmark/pdfroff.1.man:
* contrib/pic2graph/pic2graph.1.man:
* contrib/rfc1345/groff_rfc1345.7.man:
* man/groff.7.man:
* man/groff_char.7.man:
* man/groff_diff.7.man:
* man/groff_font.5.man:
* man/groff_out.5.man:
* man/groff_tmac.5.man:
* man/roff.7.man:
* src/devices/grodvi/grodvi.1.man:
* src/devices/grohtml/grohtml.1.man:
* src/devices/grolbp/grolbp.1.man:
* src/devices/grolj4/grolj4.1.man:
* src/devices/gropdf/gropdf.1.man:
* src/devices/gropdf/pdfmom.1.man:
* src/devices/grops/grops.1.man:
* src/devices/grotty/grotty.1.man:
* src/devices/xditview/gxditview.1.man:
* src/preproc/eqn/eqn.1.man:
* src/preproc/eqn/neqn.1.man:
* src/preproc/grn/grn.1.man:
* src/preproc/pic/pic.1.man:
* src/preproc/preconv/preconv.1.man:
* src/preproc/refer/refer.1.man:
* src/preproc/soelim/soelim.1.man:
* src/preproc/tbl/tbl.1.man:
* src/roff/groff/groff.1.man:
* src/roff/nroff/nroff.1.man:
* src/roff/troff/troff.1.man:
* src/utils/addftinfo/addftinfo.1.man:
* src/utils/afmtodit/afmtodit.1.man:
* src/utils/grog/grog.1.man:
* src/utils/hpftodit/hpftodit.1.man:
* src/utils/indxbib/indxbib.1.man:
* src/utils/lkbib/lkbib.1.man:
* src/utils/lookbib/lookbib.1.man:
* src/utils/pfbtops/pfbtops.1.man:
* src/utils/tfmtodit/tfmtodit.1.man:
* src/utils/xtotroff/xtotroff.1.man:
* tmac/groff_man.7.man.in:
* tmac/groff_me.7.man:
* tmac/groff_ms.7.man:
* tmac/groff_trace.7.man:
* tmac/groff_www.7.man: Do it.

Fixes <https://savannah.gnu.org/bugs/?63825>.  Thanks greatly to Alexis
for identifying an issue with mandoc(1)'s handling of an earlier
iteration of this fallback, and to John Gardner for identifying a method
of detecting mandoc as the renderer at formatting time.

Tested with groff 1.22.4, mandoc 1.14.5, and groff Git HEAD (of course).

The sed script I used follows.

/^\.cp 0/a\
.\
.\\" Define fallback for groff 1.23's MR macro if the system lacks it.\
.nr do-fallback 0\
.if !\\n(.f           .nr do-fallback 1 \\" mandoc\
.if  \\n(.g .if !d MR .nr do-fallback 1 \\" older groff\
.if !\\n(.g           .nr do-fallback 1 \\" non-groff *roff\
.if \\n[do-fallback]  \\{\\\
.  de MR\
.    ie \\\\n(.$=1 \\\
.      I \\%\\\\$1\
.    el \\\
.      IR \\%\\\\$1 (\\\\$2)\\\\$3\
.  .\
.\\}\
.rr do-fallback

ANNOUNCE: Update bug counts.  Credit Alexis with assistance.

1 files changed, 79 insertions, 0 deletions

diff --git a/ChangeLog b/ChangeLog
index 1ca54b3c9..7a7bb4a73 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,82 @@
+2023-02-21  G. Branden Robinson <g.branden.robinson@gmail.com>
+
+	[man pages]: Define page-local `MR` fallback.
+
+	[man pages]: Provide page-local fallback definition of new `MR`
+	macro.  I didn't think I was going to have to do this, but the
+	premier site on the web for viewing Linux man pages, Michael
+	Kerrisk's man7.org, has been pulling snapshots of the pages
+	themselves without upgrading the underlying macros, and so man
+	page cross references set with `MR` are getting lost in its
+	presentations.  (I acknowledge: Ingo Schwarze warned me
+	something like this could happen.)  This definition is intended
+	as a stopgap measure only.  I want to revert this after groff
+	1.23 is released and has spread to some reasonable degree.
+
+	* contrib/chem/chem.1.man:
+	* contrib/eqn2graph/eqn2graph.1.man:
+	* contrib/gdiffmk/gdiffmk.1.man:
+	* contrib/glilypond/glilypond.1.man:
+	* contrib/gperl/gperl.1.man:
+	* contrib/gpinyin/gpinyin.1.man:
+	* contrib/grap2graph/grap2graph.1.man:
+	* contrib/hdtbl/groff_hdtbl.7.man:
+	* contrib/mm/groff_mm.7.man:
+	* contrib/mm/groff_mmse.7.man:
+	* contrib/mm/mmroff.1.man:
+	* contrib/mom/groff_mom.7.man:
+	* contrib/pdfmark/pdfroff.1.man:
+	* contrib/pic2graph/pic2graph.1.man:
+	* contrib/rfc1345/groff_rfc1345.7.man:
+	* man/groff.7.man:
+	* man/groff_char.7.man:
+	* man/groff_diff.7.man:
+	* man/groff_font.5.man:
+	* man/groff_out.5.man:
+	* man/groff_tmac.5.man:
+	* man/roff.7.man:
+	* src/devices/grodvi/grodvi.1.man:
+	* src/devices/grohtml/grohtml.1.man:
+	* src/devices/grolbp/grolbp.1.man:
+	* src/devices/grolj4/grolj4.1.man:
+	* src/devices/gropdf/gropdf.1.man:
+	* src/devices/gropdf/pdfmom.1.man:
+	* src/devices/grops/grops.1.man:
+	* src/devices/grotty/grotty.1.man:
+	* src/devices/xditview/gxditview.1.man:
+	* src/preproc/eqn/eqn.1.man:
+	* src/preproc/eqn/neqn.1.man:
+	* src/preproc/grn/grn.1.man:
+	* src/preproc/pic/pic.1.man:
+	* src/preproc/preconv/preconv.1.man:
+	* src/preproc/refer/refer.1.man:
+	* src/preproc/soelim/soelim.1.man:
+	* src/preproc/tbl/tbl.1.man:
+	* src/roff/groff/groff.1.man:
+	* src/roff/nroff/nroff.1.man:
+	* src/roff/troff/troff.1.man:
+	* src/utils/addftinfo/addftinfo.1.man:
+	* src/utils/afmtodit/afmtodit.1.man:
+	* src/utils/grog/grog.1.man:
+	* src/utils/hpftodit/hpftodit.1.man:
+	* src/utils/indxbib/indxbib.1.man:
+	* src/utils/lkbib/lkbib.1.man:
+	* src/utils/lookbib/lookbib.1.man:
+	* src/utils/pfbtops/pfbtops.1.man:
+	* src/utils/tfmtodit/tfmtodit.1.man:
+	* src/utils/xtotroff/xtotroff.1.man:
+	* tmac/groff_man.7.man.in:
+	* tmac/groff_me.7.man:
+	* tmac/groff_ms.7.man:
+	* tmac/groff_trace.7.man:
+	* tmac/groff_www.7.man: Do it.
+
+	Fixes <https://savannah.gnu.org/bugs/?63825>.  Thanks greatly to
+	Alexis for identifying an issue with mandoc(1)'s handling of an
+	earlier iteration of this fallback, and to John Gardner for
+	identifying a method of detecting mandoc as the renderer at
+	formatting time.
+
 2023-02-20  G. Branden Robinson <g.branden.robinson@gmail.com>
 
 	* tmac/tests/an_TS-do-not-keep-tables-when-cR-set.sh: Improve