In a Linux user namespace that doesn't have UID 0 mapped, WriteMsgUnix
will return an EINVAL as the uid-valid-in-uns check comes first in the kernel.

Even if in a user and PID namespace, using the wrong PID in Ucred will
always give EPERM.

Change-Id: Ia7452bbf2911c3b9a2aa5d7df1572e8b0790ff38
GitHub-Last-Rev: 341d3f0cba0511411caffd2ac5a6c8dfb7a022ef
GitHub-Pull-Request: golang/sys#7
Reviewed-on: https://go-review.googlesource.com/103857
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>