This imports a simpler version of GNULIB's getpass() function
impl for Windows. Note that GNULIB's impl was buggy as it
returned a static string on UNIX, and a heap allocated string
on Windows. This new impl always heap allocates.
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>