Add sockbuf_pushsync(), an inline function that, following a change to
the head of the mbuf chains in a socket buffer, re-synchronizes the
cache pointers used to optimize socket buffer appends. This will be
used by soreceive() before dropping socket buffer mutexes to make sure
a consistent version of the socket buffer is visible to other threads.

While here, update copyright to account for substantial rewrite of much
socket code required for fine-grained locking.