Fix passwd getting error 1265 when running on newer Windows

Fix passwd getting error 1265 when running on newer Windows

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>