jail(3lua): add a jail.list() method

This is implemented as an iterator, reusing parts of the earlier logic
to populate jailparams from a passed in table.

The user may request any number of parameters to pull in while we're
searching, but we'll force jid and name to appear at a minimum.

Reviewed by: freqlabs
Differential Revision: https://reviews.freebsd.org/D26756