diagnostics: fix styling issues

Single point locations (equal boundaries) are troublesome, and we were
incorrectly ending the style in their case.  Which results in an abort
in libtextstyle.

There is also a confusion between columns as displayed on the
screen (which take into account multibyte characters and tabulations),
and the number of bytes.  Counting the screen-column
incrementally (character by character) is uneasy (because of multibyte
characters), and I don't want to maintain a buffer of the current line
when displaying the diagnostic.  So I believe the simplest solution is
to track the byte number in addition to the screen column.

* src/location.h, src/location.c (boundary): Add the byte-column.
Adjust dependencies.
* src/getargs.c, src/scan-gram.l: Adjust.
* tests/diagnostics.at: Check zero-width locations.

1 files changed, 0 insertions, 0 deletions

diff --git a/gnulib b/gnulib
-Subproject 0d8714b7ced8e0e524014c57eb884a170d62cf4
+Subproject d6af24178ca4b952327226ad64a4daad8e19109