maint: avoid locale-sensitivity in string case comparisons

strcase{cmp/str} have the drawback of being sensitive to the global
locale; this is unacceptable in a library setting.  Prefer a
hard-coded C locale alternative for all but virsh, which is user
facing and where the global locale isn't changing externally.

* .gnulib: Update to latest, for c-strcasestr change.
* bootstrap.conf (gnulib_modules): Drop strcasestr, add c-strcase
and c-strcasestr.
* cfg.mk (sc_avoid_strcase): New rule.
(exclude_file_name_regexp--sc_avoid_strcase): New exception.
* src/internal.h (STRCASEEQ, STRCASENEQ, STRCASEEQLEN)
(STRCASENEQLEN): Adjust offenders.
* src/qemu/qemu_monitor_text.c (qemuMonitorTextEjectMedia):
Likewise.
* tools/virsh.c (namesorter): Document exception.

tools/virsh.c

@@ -432,6 +432,7 @@ static int namesorter(const void *a, const void *b) {
   const char **sa = (const char**)a;
   const char **sb = (const char**)b;
 
+  /* User visible sort, so we want locale-specific case comparison.  */
   return strcasecmp(*sa, *sb);
 }