REORG/MEDIUM: move the default accept function from sockstream to protocols.c

REORG/MEDIUM: move the default accept function from sockstream to protocols.c

The previous sockstream_accept() function uses nothing from sockstream, and
is totally irrelevant to stream interfaces. Move this to the protocols.c
file which handles listeners and protocols, and call it listener_accept().

It now makes much more sense that the code dealing with listen() also handles
accept() and passes it to upper layers.