Collect sockets for leak detection

[Dan Smith]

The objhash doesn't currently collect sockets for leak detection.  The
second patch in this set adds that support for general sockets (collecting
the socket itself and any DEAD sockets referenced in their read/write
queues) as well as a proto_ops operation for a per-protocol collection
routine.

I separated out the first patch in this set to make sure it's called out
for discussion as I expect it will be contentious.  Since sockets can
have a refcount of zero while remaining valid, the assumption that the
objhash makes about the refcount (which is correct in other cases) doesn't
hold true for sockets that are DEAD and only hanging around because of
an outstanding socket buffer.