if_stf: add 6rd support

Implement IPv6 Rapid Deployment (RFC5969) on top of the existing 6to4
(RFC3056) if_stf code.

PR: 253328
Reviewed by: hrs
Obtained from: pfSense
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D33037