g13: Allow command line style "g13 mount foo".

* g13/g13.c (main): Set flag ARGPARSE_FLAG_COMMAND.
--

This requires gpgrt 1.48.  Of course "g13 --mount foo" continues to
work.

g13/g13.c

@@ -457,6 +457,9 @@ main (int argc, char **argv)
   pargs.argv  = &argv;
   pargs.flags |=  (ARGPARSE_FLAG_RESET
                    | ARGPARSE_FLAG_KEEP
+#if GPGRT_VERSION_NUMBER >= 0x013000 /* >= 1.48 */
+                   | ARGPARSE_FLAG_COMMAND
+#endif
                    | ARGPARSE_FLAG_SYS
                    | ARGPARSE_FLAG_USER);