netinet6: allow binding to anycast addresses

the restriction on sending packets from anycast source addresses was
removed in RFC4291, so there's no reason to forbid binding to such
addresses. this allows anycast services (e.g., DNS) to actually use
anycast addresses, which was previously impossible.

RFC4291 also removes the restriction that only routers may configure
anycast addresses; this was never enforced in code but was documented in
ifconfig.8. update ifconfig.8 to document both changes.

PR: 285545
Reviewed by: des, adrian
Approved by: des (mentor)
Differential Revision: https://reviews.freebsd.org/D49905