jail: Let a couple of parameter types be specified as lists

vnet.interface and zfs.dataset can be used to specify multiple
interfaces/datasets in jail.conf, but not on the command-line, which is
a bit surprising. Extend the handling of ip(4|6).addr to those
parameters, update the description of vnet.interface in jail.8, and add
a rudimentary regression test.

Reviewed by: zlei, jamie
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D47651