Add support for non-contiguous IPv6 masks in ipfw(8) rules.

For example fe::640:0:0/ffff::ffff:ffff:0:0 will match
addresses fe:*:*:*:0:640:*:*

Submitted by:	Eugene Mamchits <mamchits at yandex-team dot ru>
Obtained from:	Yandex LLC
Sponsored by:	Yandex LLC