CL 114518 introduced Nfgenmsg as a type alias which breaks the build on
Go <= 1.9. Fix it by making Nfgenmsg a type like all others in
x/sys/unix

Updates golang/go#25561
Fixes golang/go#25563

Change-Id: I9950be857e34e7e3ca79c71fced9663a0cac9b63
Reviewed-on: https://go-review.googlesource.com/114596
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>