Add three new socket status functions:

socket_readable() returns TRUE if recv() will not block if called
on the specified socket.

socket_writable() returns TRUE if send() will not block if called
on the specified socket.

socket_recvdone() returns TRUE if all data has been recv()ed an
the socket is closed.

"will not block" includes "will return an error".

This matches the three main use-cases for select() of a single socket
in Synchronet.

Use them in the SyncTERM RLogin stuff so they can be tested easily.