This commit adds AIX operating system to internal/nettest package.

It also disables unixpacket network.

Change-Id: Ie1a0b5c02d41ede7297c3706f6967be3438321a5
Reviewed-on: https://go-review.googlesource.com/c/144077Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>