swapon: more robust progname probing

Currently, the 'swapoff' is the default behavior. It means that if you
have a libtool wrapper script in your build directory the executed binary
name is 'lt-swapon' and then "swapon -a" is interpreted as "swapoff -a".

Any random argv[0] should not be interpreted as "swapoff".

Signed-off-by: Karel Zak <kzak@redhat.com>

mount/swapon.c

@@ -791,6 +791,9 @@ main(int argc, char *argv[]) {
 
 	if (streq(progname, "swapon"))
 		return main_swapon(argc, argv);
-	else
+	else if (streq(progname, "swapoff"))
 		return main_swapoff(argc, argv);
+
+	errx(EXIT_FAILURE, _("'%s' is unsupported program name "
+			"(must be 'swapon' or 'swapoff')."), progname);
 }