if_gif(4): Support the NOCLAMP flag to change MTU handling for IPv6

The patch was originally written by hrs [1], and later modified by meta
to use named flags instead of generic link-layer flags.

[1] https://reviews.freebsd.org/D45854

PR: 280736
Co-authored-by: Hiroki Sato <hrs@FreeBSD.org>
Reviewed by: ae, ziaee, zlei, pauamma
Reported by: Kazuki Shimizu <kazubu@jtime.net>
Approved by: pauamma (manpages)
Approved by: ae
MFC after: 2 weeks
Sponsored by: Cybertrust Japan
Differential Revision: https://reviews.freebsd.org/D51297