The current interface can't access all environment
variables directly or via cgi.RequestFromMap, which
only reads variables on its "white list" to be set on
the http.Request it returns. If an fcgi variable is
not on the "white list" - e.g. REMOTE_USER - the old
code has no access to its value.

This passes variables in the Request context that aren't
used to add data to the Request itself and adds a method
that parses those env vars from the Request's context.

Fixes #16546

Change-Id: Ibf933a768b677ece1bb93d7bf99a14cef36ec671
Reviewed-on: https://go-review.googlesource.com/40012
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>