Reduce remote.c/x11.c coupling.

What used to be the global `remote_params` is now just passed in
to `platform_init()`.
Comments on various things.

Doesn't cover main.c; will hopefully get to that.
Do away with `schedule_message()`.

It was kind of a needless specialization.
Convert wire protocol to (mostly) XDR.

Eliminates the tedious boilerplate crap in proto.c (the remainder of
which is now known as message.c) and centralizes protocol information
in one place (proto.x).

There's still one field of "manual" format conversion (via htonl(3),
ntohl(3)); see comment in message.c for explanation.