Continuing the procket clean up

Some small changes to procket. If you're playing along at home, the changes don't affect the procket:listen/1 and procket:listen/2 interface.

The procket NIF now requires the elements to be acted on (the path to the Unix socket and/or the file descriptor of the listener on the Unix socket) to be explicitly passed. The Erlang code must track the path to the Unix socket, the fd listening on the Unix socket and the privileged socket returned from procket_cmd.

Since the procket NIF does not internally track any state or allocate memory, it can handle multiple file desciptors up to the process' resource limits.

The changes from the commit message:

open/2 -> open/1 : vestigal protocol arg removed, stick to streams. Erlang module changed to match (along with the bizarre passing in of the port as a protocol, who did that? o_O)

Returns the socket descriptor listening on the Unix socket: {ok, FD}

poll/0 -> poll/1 : takes the socket descriptor, returns {ok, Socket}

close/0 -> close/2 : close(SocketPath, SocketDescriptor), closes the socket descriptor and deletes the socket path, returns "ok"