Add support for a /etc/defaults/vendor.conf override file

Reviewed by:	stas, imp
Approved by:	re (gjb)
Differential Revision:	https://reviews.freebsd.org/D6895

etc/defaults/rc.conf

@@ -741,3 +741,9 @@ if [ -z "${source_rc_confs_defined}" ]; then
 		done
 	}
 fi
+
+# Allow vendors to override FreeBSD defaults in /etc/default/rc.conf
+# without the need to carefully manage /etc/rc.conf.
+if [ -r /etc/defaults/vendor.conf ]; then
+	. /etc/defaults/vendor.conf
+fi