Add ipfw_nptv6 module that implements Network Prefix Translation for IPv6

Add ipfw_nptv6 module that implements Network Prefix Translation for IPv6
as defined in RFC 6296. The module works together with ipfw(4) and
implemented as its external action module. When it is loaded, it registers
as eaction and can be used in rules. The usage pattern is similar to
ipfw_nat(4). All matched by rule traffic goes to the NPT module.

Reviewed by:	hrs
Obtained from:	Yandex LLC
MFC after:	1 month
Relnotes:	yes
Sponsored by:	Yandex LLC
Differential Revision:	https://reviews.freebsd.org/D6420