Use sockbuf_pushsync() to synchronize stack and socket buffer state

in soreceive() after removing an MT_SONAME mbuf from the head of the
socket buffer.

When processing MT_CONTROL mbufs in soreceive(), first remove all of
the MT_CONTROL mbufs from the head of the socket buffer to a local
mbuf chain, then feed them into dom_externalize() as a set, which
both avoids thrashing the socket buffer lock when handling multiple
control mbufs, and also avoids races with other threads acting on
the socket buffer when the socket buffer mutex is released to enter
the externalize code.  Existing races that might occur if the protocol
externalize method blocked during processing have also been closed.

Now that we synchronize socket buffer and stack state following
modifications to the socket buffer, turn the manual synchronization
that previously followed control mbuf processing with a set of
assertions.  This can eventually be removed.

The soreceive() code is now substantially more MPSAFE.

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