Execute the start/stop process of a jail in the background.

This will prevent that the script hangs during startup, which
could cause annoying effects after rebooting for example.

PR:		kern/139422
Submitted by:	Andrey Groshev <greenx at yartv dot ru>
Approved by:	imp (mentor, implicit)
MFC after:	3 days
Facilitated by:	Snow B.V.

etc/rc.d/jail

@@ -728,4 +728,4 @@ fi
 if [ -n "$*" ]; then
 	jail_list="$*"
 fi
-run_rc_command "${cmd}"
+run_rc_command "${cmd}" &