tcp: Rack not being very friendly with V6:4 socket and having a connection from V4

There were two bugs that prevented V4 sockets from connecting to
a rack server running a V4/V6 socket. As well as a bug that stops the
mapped v4 in V6 address from working.

Reviewed by: mtuexen
Sponsored by: Netflix Inc
Differential Revision: https://reviews.freebsd.org/D30885