On Windows 8.1 and later, the NetUserChangePassword call apparently
doesn't accept the usual "\\server" string anymore, but requires to
use the "domain" instead, otherwise it emits en error code 1265,
ERROR_DOWNGRADE_DETECTED. Since this is accepted by pre-8.1 as well,
use the domain indiscriminately when calling NetUserChangePassword
from passwd(1).
While at it, do some minor cleanup in passwd.c.
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>