Added support for extra ifconfig args to jail ip4.addr & ip6.addr params
This allows for CARP interfaces to be used in jails e.g.
ip4.addr = "em0|10.10.1.20/32 vhid 1 pass MyPass advskew 100"
r269340 will not be MFC'ed as mentioned due to the slim window and the
amount of additional commits required to support it.
Sponsored by: Multiplay