Provides access to the GetComputerNameEx syscall.

This mirrors the change golang.org/cl/5852 in the standard library, but
provides public access to all name types provided by windows.

Change-Id: I5fbad5abe721de70e9d2b5dda2fafb7a9c419220
Reviewed-on: https://go-review.googlesource.com/6320Reviewed-by: Alex Brainman <alex.brainman@gmail.com>