net: don't return still-connecting fd.

If one connect failed, we'd return the other one without
waiting for the connect to complete.  This resulted in
read() returning 0, which was really weird.

The downside: the poll doesn't seem to time out when the
connect times out (Linux 3.13 x86-64).

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