MFC jail: Remove a prison's shared memory when it dies

Add shm_remove_prison(), that removes all POSIX shared memory segments
belonging to a prison. Call it from prison_cleanup() so a prison
won't be stuck in a dying state due to the resources still held.

PR: 257555
Reported by: grembo

(cherry picked from commit 7060da62ff18e8e52c5e41f0794cc4f10dadfc6e)