pkill(1): add '-j jid' flag to restrict matches to jailed processes.

pkill(1): add '-j jid' flag to restrict matches to jailed processes.

Add a '-j' option to pgrep(1)/pkill(1).
Update manual page to reflect usage.

As DragonFlyBSD did not have this option previously, the 'COMPATIBILITY'
notes outlined in FreeBSD's '-j' options do not apply - namely,
the '-j 0' option defaults to non-jailed processes, and '-1' is used
to indicate all jailed processes should be matched. Also, the 'any'/'none'
string options were not ported as they require more invasive changes.

Inspired-by: FreeBSD