net/http/cgi: Correctly pass down the REMOTE_PORT value for CGI requests.

Currently when we get a CGI or FCGI request, the remote port of the client
is hard coded to zero, despite nearly every webserver passing down the
REMOTE_PORT variable.

This was likely originally excluded because the CGI RFC (rfc3875) does not
mention anything about the remote port of the client. However every webserver
tested does pass REMOTE_PORT down. This includes Apache 2.2, Apache 2.4,
nginx and lighttpd.

Fixes #8351

Change-Id: I4c6366cb39f0ccc05e038bd31d85f93b76e8d0c8
Reviewed-on: https://go-review.googlesource.com/1750


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>