ccan/io: have io_plan mappable back to io_conn.

io_conn contains two plans: by knowing what plan this is, it can be cast
back into the io_conn.  That helps for the next patch.

We also remove the strange logic where io_duplex() would return a
magic, invalid io_plan pointer.

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