Make openup -c mutually exclusive with other options.

openup

@@ -37,13 +37,14 @@ _OPENUP_MINREL=53
 
 usage() {
 	echo
-	echo "Usage: ${0##*/} [-SUn]" >&2
+	echo "Usage: ${0##*/} [-SUn]|[-c]" >&2
 	echo
 	echo "Options:"
 	echo "  -S    do not check for package signatures"
 	echo "  -U    do not check for openup update"
-	echo "  -c    check/cron mode, other options are ignored"
 	echo "  -n    dry-run mode, no modification is done"
+	echo
+	echo "  -c    check/cron mode, mutually exclusive with other options"
 	exit 1
 }
 
@@ -215,7 +216,11 @@ if [ -z "${checkmode}" ]; then
 	update_binpatches
 	update_pkg
 else
-	check_vuxml
+	if [ "${nosig}" -o "${noselfupdate}" -o "${dryrun}" ]; then
+		usage
+	else
+		check_vuxml
+	fi
 fi
 
 rm -f ${_TMPDIR}/openup-*