In particular, the main daemon and subdaemons share the backtrace code,
with hooks for logging.
The daemon hook inserts the io_poll override, which means we no longer
need io_debug.[ch]. Though most daemons don't need it, they still link
against ccan/io, so it's harmess (suggested by @ZmnSCPxj).
This was tested manually to make sure we get backtraces still.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Rusty Russell