dev_blackhole_fd was a hack, and doesn't work well now we are async
(it worked for sync comms in per-peer daemons, but now we could sneak
through a read before we get to the next write).
So, make explicit flags and use them. This is much easier now we
have all peer comms in one place.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>