We make it a macro, since everyone uses PEER_FD and GOSSIP_FD constants
(they're actually always the same, but this is slightly safer), and
add a gossip_index arg: this is groundwork for when we want to hand
the peer back to master for gossipd.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
This change is really to allow us to have a --dev-fail-on-subdaemon-fail option
so we can handle failures from subdaemons generically.
It also neatens handling so we can have an explicit callback for "peer
did something wrong" (which matters if we want to close the channel in
that case).
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>