netinet6: Remove support for connecting to IN6ADDR_ANY

[emaste]

Summary

RFC4291 section 2.5.2:

The unspecified address must not be used as the destination address
of IPv6 packets or in IPv6 Routing headers.  An IPv6 packet with a
source address of unspecified must never be forwarded by an IPv6
router.

We disallowed connections to IN6ADDR_ANY by default, as of commit 627e126dbb07b167b028380ef61bb45f10493938. As this is actually disallowed by the RFC, just remove support.

Reported by: bz (in D54306)

[bz]

Seems fine to me. Do we have a test case for this (which may need to be adjusted)?

[glebius]

sys/netinet6/in6_pcb.c

391–392

Let's add comment /* RFC4291 section 2.5.2 */ right above this line.