jail: add jail descriptors

Similar to process descriptors, jail desriptors are allow jail
administration using the file descriptor interface instead of JIDs.
They come from and can be used by jail_set(2) and jail_get(2),
and there are two new system calls, jail_attach_jd(2) and
jail_remove_jd(2).

Reviewed by: bz, brooks
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D43696