internal/nettest: add SupportsRawIPSocket that reports raw IP socket availability

This change copies a few code fragments from
https://golang.org/cl/13422044.

Change-Id: I7ceae3a59e15dc9f9ef4b2b64a41753aa9d6798b
Reviewed-on: https://go-review.googlesource.com/3400
Reviewed-by: Ian Lance Taylor <iant@golang.org>
3 files changed