Separate POSIX sem/shm objects in jails, by prepending the jail's path
name to the object's "path".  While the objects don't have real path
names, it's a filesystem-like namespace, which allows jails to be
kept to their own space, but still allows the system / jail parent to
access a jail's IPC.

Separate POSIX mqueue objects in jails; actually, separate them by the
jail's root, so jails that don't have their own filesystem directory
also won't have their own mqueue namespace.

Clean up some style(9) violations.

Use the new PR_METHOD_REMOVE to clean up jail handling in POSIX
message queues.