When looking for a local username on Windows, if we can get hold of the

When looking for a local username on Windows, if we can get hold of the
NameUserPrincipal, use that; this avoids an issue with SSPI/GSSAPI where
the user logged in to the local machine with a different case of username
to the (case-sensitive) Kerberos username. Falls back to GetUserName as
before if that doesn't work (for machines not on a domain, and Win9x).
Based on a patch by SebastianUnger.

[originally from svn r8909]