pseudotcp: Also cause Writable callback to be emitted if can_send() says no

The GOutputStream based systems can sometimes call this, decide the stream is
not writable and then poll for it. We have to make sure that poll is woken up.
No need to strdup foundation in new_selected_pair since we queue signal
agent: Don't return the socket to the app in any reliable case
tcp-bsd: Don't clear error that hasn't been set
Small documentation fix
Add ICE-TCP candidate transport types
Fix documentation in preparation for ice-tcp support
Some refactoring to allow for ice-tcp (non pseudotcp reliable)
Add a socket type for socket layer
Allow tcp-bsd to act as reliable or non reliable transport and fix is_reliable on other sockets
Fix return value of send_messages in socks5
Return number of sent messages in socket_send_messages when an error happens part-way
turn: In case of error, don't just pass through the data, return an error
Add a nice_socket_send_reliable API for internal use.
One issue with tcp-bsd is that it will queue messages when the tcp socket
is not writable, but it will also drop messages when the queue is full.
If we want to do proper reliable ice-tcp, we need to make sure that
messages don't get dropped, also, this would affect http/socks5/pseudossl
if their messages get dropped. For ice-tcp, when the socket is not writable
we want to return 0, not queue the message.
The change here is to allow connchecks and other important protocol messages
to be sent as 'reliable' on tcp transports by queuing them and in the various
socket layers and to never drop them, but all user messages will be dropped.

if the tcp socket is not writable.
Factor out pseudotcp so it's not used if the transport is already reliable
Add support for MS Office Communicator pseudossl compatibility
Add support for MS-TURN in tcp-turn
Small refactoring of agent, loop on component id from 1
Fix indentation for _set_remote_candidates_locked
Refactor discovery/agent to add support for non-udp candidates