coio: return fd from coio_connect

coio_connect sets fd in ev_io passed to it. Let's return fd explicitly
and remove the ev_io argument (we can use coio_wait, which creates a
temporary ev_io object, inside coio_connect - it's cheap).

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.