maint: address a couple of syntax-check errors.

* cfg.mk (local-checks-to-skip): Remove sc_error_message_period
	and sc_error_message_uppercase.
	Address the uncovered issues.
	* po/POTFILES.in: Add missing files.
	* src/symtab.c: Remove useless includes.
	* lib/bitset_stats.c, src/files.c, tests/glr-regression.at: Use
	conformant error messages.

src/files.c

@@ -127,7 +127,7 @@ xfclose (FILE *ptr)
     return;
 
   if (ferror (ptr))
-    error (EXIT_FAILURE, 0, _("I/O error"));
+    error (EXIT_FAILURE, 0, _("input/output error"));
 
   if (fclose (ptr) != 0)
     error (EXIT_FAILURE, get_errno (), _("cannot close file"));

src/symtab.c

@@ -22,11 +22,9 @@
 #include "system.h"
 
 #include <hash.h>
-#include <quotearg.h>
 
 #include "complain.h"
 #include "gram.h"
-#include "quote.h"
 #include "symtab.h"
 
 /*-------------------------------------------------------------------.