Allow implicit TCP connection setup for TCP/IPv6.

TCP/IPv4 allows an implicit connection setup using sendto(), which
is used for TTCP and TCP fast open. This patch adds support for
TCP/IPv6.
While there, improve some tests for detecting multicast addresses,
which are mapped.

Reviewed by: bz@, kbowling@, rrs@
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D16458