tree-wide: reset optind to 0 when GNU extensions in optstring are used

Otherwise, if getopt() and friends are used before parse_argv(), then
the GNU extensions may be ignored.

This should not change any behavior at least now, as we usually use
getopt_long() only once per invocation. But in the next commit,
getopt_long() will be used for other arrays, hence this change will
become necessary.

src/userdb/userdbctl.c

@@ -1150,6 +1150,10 @@ static int parse_argv(int argc, char *argv[]) {
                 arg_services = l;
         }
 
+        /* Resetting to 0 forces the invocation of an internal initialization routine of getopt_long()
+         * that checks for GNU extensions in optstring ('-' or '+' at the beginning). */
+        optind = 0;
+
         for (;;) {
                 int c;