netinet6: Disallow connections to IN6ADDR_ANY

[emaste]

Summary

Previously connect() or sendto() to INADDR_ANY or IN6ADDR_ANY reached
some socket bound to some host interface address.  Although this was
intentional it was an artifact of a different era, and is not desirable
now.

In 417b35a97b76 markj added support to disallow connect() to INADDR_ANY
and IN6ADDR_ANY.  Connections to INADDR_ANY were disabled by default in
cd240957d7ba.  Follow suit with IN6ADDR_ANY.

[markj]

This should be run through the test suite, in case tests are relying on this behaviour.

[emaste]

This should be run through the test suite, in case tests are relying on this behaviour.

Indeed, will check at least that before pushing. After a quick grep I didn't find much of interest and I imagine a failure is unlikely; (2 copies of) libevent/test/regress_util.c was all that I spotted that seems somewhat interesting.

[zlei]

This is a behavior change. I think it deserves a Relnotes: Yes commit meta message.

[glebius]

I'd also suggest to change it to SYSCTL_BOOL.