change more quotes in source, and adjust tests to match

Run this command to change each `%s' to '%s' in source directories:
  git grep -l '`%s'\' src djgpp data \
    |xargs perl -pi -e '$q="'\''";s/`%s$q/$q%s$q/g'
* data/bison.m4: Affected per the above.
* djgpp/subpipe.c: Likewise.
* src/files.c: Likewise.
* src/getargs.c: Likewise.
* src/muscle-tab.c: Likewise.
* src/reader.c: Likewise.
* tests/glr-regression.at: Adjust to match.
* tests/input.at: Likewise.
* tests/push.at: Likewise.
* tests/skeletons.at: Likewise.

src/files.c

@@ -109,7 +109,7 @@ xfopen (const char *name, const char *mode)
 
   ptr = fopen_safer (name, mode);
   if (!ptr)
-    error (EXIT_FAILURE, get_errno (), _("cannot open file `%s'"), name);
+    error (EXIT_FAILURE, get_errno (), _("cannot open file '%s'"), name);
 
   return ptr;
 }