r3304: changed the API to lib/socket/ a little.

r3304: changed the API to lib/socket/ a little.

The main change is to make socket_recv() take a pre-allocated buffer,
rather than allocating one itself. This allows non-blocking users of
this API to avoid a memcpy(). As a result our messaging code is now
about 10% faster, and the ncacn_ip_tcp and ncalrpc code is also
faster.

The second change was to remove the unused mem_ctx argument from
socket_send(). Having it there implied that memory could be allocated,
which meant the caller had to worry about freeing that memory (if for
example it is sending in a tight loop using the same memory
context). Removing that unused argument keeps life simpler for users.
(This used to be commit a16e4756cd68ca8aab4ffc59d4d9db0b6e44dbd1)