MFC r265229,r265239:

    Add -J to filter by matching jail IDs and names.
MFC r265249,r265250,r265251:

    - Add -J command/flag to filter by jail name/jid. This will automatically
      display the JID as well (the -j command/flag).
    - Add a hint for 'u' and 'J' command that '+' displays all.
    - Add J command to help.