jail: add kqueue(2) support for jails

Add kqueue tracking to jails, inspired by how it's done with processes.
EVFILT_JAIL takes a jail ID, and tracks with NOTE_JAIL_SET,
NOTE_JAIL_ATTACH, NOTE_JAIL_REMOVE, and NOTE_JAIL_CHILD. It also uses
the NOTE_TRACK mechanism that EVFILT_PROC uses, using the same result
flags (NOTE_CHILD and NOTE_TRACKERR).

Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D51940