Make internet PCB connect more consistent.

Make internet PCB connect more consistent.

The public interface is in_pcbconnect().  It dispatches to
in6_pcbconnect() if necessary.  Call the former from tcp_connect()
and udp_connect().
In in6_pcbconnect() initialization in6a = NULL is not necessary.
in6_pcbselsrc() sets the pointer, but does not read the value.
Pass a constant in6_addr pointer to in6_pcbselsrc() and in6_selectsrc().
It returns a reference to the address of some internal data structure.
We want to be sure that in6_addr is not modified this way.  IPv4
in_pcbselsrc() solves this by passing a copy of the address.

OK kn@ sashan@ mvs@