Assert that the corresponding socket is locked when manipulating socket

Assert that the corresponding socket is locked when manipulating socket
buffers.

This is one step towards unlocking TCP input path.  Note that all the
functions asserting for the socket lock are not necessarilly MP-safe.
All the fields of 'struct socket' aren't protected.

Introduce a new kernel-only kqueue hint, NOTE_SUBMIT, to be able to
tell when a filter needs to lock the underlying data structures.  Logic
and name taken from NetBSD.

Tested by Hrvoje Popovski.

ok claudio@, bluhm@, mikeb@