On Solaris, if you do a in-progress connect, and then the
server accepts and closes the socket, the client's later
attempt to complete the connect will fail with EINVAL.  Handle
this case by assuming that the connect succeeded.  This code
is weird enough that it is implemented as Solaris-only so that
it doesn't hide a real error on a different OS.

Update #6828

R=golang-codereviews, bradfitz, dave
CC=golang-codereviews
https://golang.org/cl/46160043