This change introduces an internal package that provides the
manipulation of net package facilities by using runtime reflection.
The package is supposed to be used by ipv4 and ipv6 packages.

Change-Id: I73ec3b7d3762e675ca03ad9ee5e8a68e75ceb997
Reviewed-on: https://go-review.googlesource.com/27732
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>