Changes

RISC OS: add SocketWatch module to distribution

RISC OS: use SocketWatch for socket activity
Use SocketWatch to monitor our sockets and convert activity into a
pollword update. This allows us to ditch the schedule-based fetch path
and take the same one as we do on other platforms. This also allows us
to refactor the main poll loop such that we need only poll idle while
waiting for things to happen (once something has happened, we poll
aggressively until there are no non-pollword events left).