net/http/cgi: fix REMOTE_ADDR, REMOTE_HOST, add REMOTE_PORT

net/http/cgi: fix REMOTE_ADDR, REMOTE_HOST, add REMOTE_PORT

Env vars were incorrectly copying whole value of http.RemoteAddr
to REMOTE_ADDR and REMOTE_HOST. They contained IP:port pair which
instead should only have IP (RFC 3875, other sources).

Module also was not setting REMOTE_PORT variable which become de-facto
standard for passing TCP client port to CGI scripts (Apache mod_cgi,
IIS, and probably others)

Fixes #9861

Change-Id: Ia73e664c48539e3c7db4997d09d957884e98d8a5
Reviewed-on: https://go-review.googlesource.com/4933
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>