Common receive buffer management for sockets

[netbsduser]

UDP, TCP, and Unix sockets all reduplicate a packet-based receive buffer mechanism. It's worth turning this into a reusable abstraction and making them all use it.

It might be more difficult to make this work with Unix sockets where SCM_RIGHTS and SCM_CREDS are present or it might not be.

[netbsduser]

Now packet sockets do too.