ccan/io: add io_out_wait() and io_out_always().

ccan/io: add io_out_wait() and io_out_always().

This specificity is required, for example, when doing:

	return io_duplex(conn, io_read(...), io_always(...));

The workaround suggested doesn't work, because io_duplex_prepare()
asserts() if the plans aren't UNSET to start.  And pettycoin needs this.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>