netinet6: allow binding to anycast addresses

[ivy]

Summary

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

[des]

sbin/ifconfig/ifconfig.8

31

it's nbd if you don't but try to remember to bump before pushing

452

We should probably add RFC 4291 (and RFC 3484 which is mentioned elsewhere in the page) to the “SEE ALSO” section. See tcp(4) or protocols(5) for examples of how to format the references.

456

overly verbose, “explicitly binds [a socket] to the address” is enough

[ivy]

fixes for ifconfig.8 review comments:

• be less verbose
• add RFCs to see also
• bump Dd