Performing multiple connect system calls on a non-blocking socket
under DragonFly BSD does not necessarily result in errors from earlier
connect calls being returned, particularly if we are connecting to
localhost. Instead, once netpoll tells us that the socket is ready,
get the SO_ERROR socket option to see if the connection succeeded
or failed.

Fixes #7474

LGTM=mikioh.mikioh
R=mikioh.mikioh
CC=golang-codereviews
https://golang.org/cl/69340044