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.

src/getargs.c

@@ -577,7 +577,7 @@ command_line_location (void)
 {
   location res;
   /* "<command line>" is used in GCC's messages about -D. */
-  boundary_set (&res.start, uniqstr_new ("<command line>"), optind - 1, -1);
+  boundary_set (&res.start, uniqstr_new ("<command line>"), optind - 1, -1, -1);
   res.end = res.start;
   return res;
 }