Add new rc.conf parameter "jail_reverse_stop"

When a user defines "jail_list" in rc.conf the jails are started in the
order defined. Currently the jails are not are stopped in reverse order
which may break dependencies between jails/services and prevent a clean
shutdown. The new parameter "jail_reverse_stop" will shutdown jails in
"jail_list" in reverse order when set to "YES".

Please note that this does not affect manual invocation of the jail rc
script. If a user runs the command

  # service jail stop jail1 jail2 jail3

the jails will be stopped in exactly the order specified regardless of
jail_reverse_stop being defined in rc.conf.

Document the new jail_reverse_stop parameter

While here clean up the documentation for jail_list