On Android devices where the stub fallback for Current fails to
extract a User from the environment, return a dummy fallback instead
of failing.

While we're here, use / instead of /home/nacl for the NaCL fallback.

Hopefully fixes the Android builder.

Change-Id: Ia29304fbc224ee5f9c0f4e706d1756f765a7eae5
Reviewed-on: https://go-review.googlesource.com/37960
Run-TryBot: Elias Naur <elias.naur@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>