The new implementation parses the plan9 interface files
and builds Interface representations for the net package.

Updates #17218

Change-Id: I3199d18d9e96a17e922186c3abff1d7cd9cbec2e
Reviewed-on: https://go-review.googlesource.com/29963
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>