MFC r269522

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.

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