coio: pass fd to coio_accept

coio_accept uses fd from ev_io passed to it. Let's pass fd explicitly
and use coio_wait for waiting for the socket to become ready.

This is a step towards hiding all ev_io manipulations in coio internals
so that coio users don't have to deal with ev_io directly.

Drop unused coio_bind declaration while we are at it (there's no
function definition).