ip6: add SO_BINTIME support

This adds support for obtaining timestamps from IPv6 packets using the
SO_BINTIME socket option, bringing it in parity with IPv4 behavior.

Enable testing the SO_BINTIME option in the relevant (manual) regression
test.

PR: 289423
Reviewed by: markj
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D52504

(cherry picked from commit cd02a8a9f8be2085d5242606a79668dc3720e7b0)