Add a kern.ipc.posix_shm_list sysctl.

The sysctl provides the listing on named linked posix shared memory
segments existing in the system.

Reuse shm_fill_kinfo() for filling individual struct kinfo_file.
Remove unneeded lock around reading of shmfd->shm_mode.

Reviewed by: jilles, tmunro
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D20258