Implement new read-only register ".nm".

It reports the enablement status of output line numbering (caused by the
.nm request) irrespective of the temporary suspension of numbering with
the .nn request.  Needed because there was no way to introspect its
state, tbl(1) needs to do so, and the writable line number register
\n[ln] is not a reliable proxy for it.

* src/roff/troff/env.h: Add get_numbering_nodes(), returning `int`,  to
  public interface of `environment` class.
* src/roff/troff/env.cpp: Implement get_numbering_nodes().

* src/roff/groff/tests/dot-nm_register_works.sh: Test it.
* src/roff/groff/groff.am (groff_TESTS): Run test.

* doc/groff.texi (Miscellaneous):
* man/groff_diff.7.man (Language/New number registers):
* man/groff.7.man (Registers/Read-only registers): Document it.

* NEWS: Add item.

1 files changed, 8 insertions, 1 deletions

diff --git a/NEWS b/NEWS
index eae82e10d..0510fd43d 100644
--- a/NEWS
+++ b/NEWS
@@ -1,4 +1,4 @@
-    Copyright (C) 1992-2020 Free Software Foundation, Inc.
+    Copyright (C) 1992-2021 Free Software Foundation, Inc.
 
     Copying and distribution of this file, with or without modification,
     are permitted in any medium without royalty provided the copyright
@@ -18,6 +18,13 @@ o A new read-only register ".cp" is implemented.  Within a .do request,
   groff_diff(7) or the groff Texinfo manual for rationale, use case, and
   example.
 
+o A new read-only register ".nm" is implemented.  It reports the
+  enablement status of output line numbering (caused by the .nm
+  request) irrespective of the temporary suspension of numbering with
+  the .nn request.  This register was introduced because there was no
+  way to introspect its state, tbl(1) needs to be able to do so, and the
+  writable line number register \n[ln] is not a reliable proxy for it.
+
 o Point-size escapes of the form '\sNN', where NN is in the range 10-39,
   are now recognized only in compatibility mode (groff -C); when
   encountered, an error diagnostic is emitted.  In normal mode, \sN is