Add jail_attach support.

Add jail_attach support.

Inspired-by: FreeBSD
Manpage-by: Paul Herman
Add sysctl jail.list and jailed() helper functions.

Obtained-from: FreeBSD

Add jexec and jls.

Obtained-from: FreeBSD

Move reference counting into prison_hold / prison_free functions.
Fix an incorrect increment in fork1, the prison structure is not
directly copied and the cr reference counting applies to prison here
too. Begin moving explicit pointer checks for cr_prison != NULL
to the new jailed() function. Move the sys/varsym.h include in
sys/jail.h to the kernel part. Prepare for jail-local securelevel.