Expanded connection-setup sequence.

Now instead of just the remote immediately sending back a READY
message, the master first initiates with a SETUP message carrying
configuration (none at the moment, but for future use), to which
the remote then sends the READY in response.
Convert remote-side fds to O_NONBLOCK.

Communications are now somewhat more abstracted behind a common
message channel ('msgchan') interface.
Integrate platform_event_fd into remote's select() loop.
Avoid a potential assert() failure.

If a remote gets failed on a read, don't attempt to write to it in
the same iteration of the select() loop.
Provide CLIPBOARD as well as PRIMARY selection.

What a flustercuck.