MFC jaildesc: remove file-mode-based access controls

Jail descriptors were given a file-like mode, user, and group, for the
purpose of controlling how the descriptor may be used. This is too far
removed from the file paradigm to make sense. Remove it in favor of a
better access control method to be added, such as Capsicum.

Also add missing code in jaildesc_fill_kinfo.

Reported by: crest at rlwinm.de, kib
MFC after: 3 days

(cherry picked from commit d81b337d690c971d60c731494795ee4b81fb929e)