Fix a jail parameter marshal error - don't panic on empty slice.
Fix a parameter panic -- jid must be passed by-pointer to be addressable
Add semi-broken support for interacting with jail network interfaces

This patch adds Jail.IpAddrs and Jail.SetIpAddrs which get/set the IP
addresses exposed to the jail. Unfortunately, there's a bug in the
IP address marshalling code when multiple IP addresses are assigned --
only the first one is actually assigned. Additional calls to SetIpAddrs
will overwrite the old value, as designed.

No idea why additional IP addresses aren't sticking.

As an additional bug, Jail.Refresh doesn't pick up IPv6 addresses, so
Jail.IpAddrs will not properly return IPv6 addresses that belong to the
jail.