Allow TCP to reuse local port with different destinations

Previously, tcp_connect() would bind a local port before connecting,
forcing the local port to be unique across all outgoing TCP connections
for the address family. Instead, choose a local port after selecting
the destination and the local address, requiring only that the tuple
is unique and does not match a wildcard binding.

Reviewed by: tuexen (rscheff, rrs previous version)
MFC after: 1 month
Sponsored by: Forcepoint LLC
Differential Revision: https://reviews.freebsd.org/D24781