Jail_ip and jail_interface local variables were renamed to _ip and _interface

in a previous commit to avoid namespace collisions, unfortunately I missed two
of them. This leads to the ip alias being incorrectly removed in some cases
when using the stop command.

Reported by:	Philipp Wuensche <cryx-freebsd@h3q.com>

etc/rc.d/jail

@@ -262,8 +262,8 @@ jail_start()
 			echo ${_jail_id} > /var/run/jail_${_jail}.id
 		else
 			jail_umount_fs
-			if [ -n "${jail_interface}" ]; then
-				ifconfig ${jail_interface} -alias ${jail_ip}
+			if [ -n "${_interface}" ]; then
+				ifconfig ${_interface} -alias ${_ip}
 			fi
 			echo " cannot start jail \"${_jail}\": "
 			tail +2 ${_tmp_jail}