s3: Basic infrastructure for /tmp/.nmbd/unexpected

s3: Basic infrastructure for /tmp/.nmbd/unexpected

This provides the framework to replace the unexpected.tdb file. Nmbd will
listen on /tmp/.nmbd/unexpected. A client interested in unexpected packets
connects there. It sends a nb_packet_query plus a potential mailslot name for
dgram packets. It waits for a single ack byte to avoid races. After that has
happened, nmbd will pass down all matching packets through that socket.

nb_packet_server_create and nb_packet_dispatch are the nmbd routines,
nb_packet_reader_send/recv and nb_packet_read_send/recv are the client ones.