In https://golang.org/cl/28951 I cleaned up the lookupProtocol and
lookupPort paths to be consistently case-insensitive across operating
systems and to share the same baked-in maps of port & proto values
that can be relied on to exist on any platform.

I missed the fallback to the baked-in maps on Windows, though, which
broke Windows XP. This should fix it.

Fixes #17175

Change-Id: Iecd434fb684304137ee27f5521cfaa8c351a1bde
Reviewed-on: https://go-review.googlesource.com/29968
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>