net: make LookupPort and lookupProtocol work on nacl

net: make LookupPort and lookupProtocol work on nacl

Also, flesh out the baked-in /etc/services table for LookupPort a bit.

This services map moves from a unix-specific file to a portable file
where nacl can use it.

Also, remove the duplicated entries in the protocol map in different
cases, and just canonicalize the input before looking in the map. Now
it handles any case, including MiXeD cAse.

In the process, add a test that service names for LookupPort are case
insensitive. They were on Windows, but not cgo. Now there's a test and
they're case insensitive in all 3+ paths. Maybe it breaks plan9. We'll
see.

Fixes #17045

Change-Id: Idce7d68703f371727c7505cda03a32bd842298cd
Reviewed-on: https://go-review.googlesource.com/28951
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>