Stop supporting the historical way of starting ypbind(8); that is setting
'ypbind_flags=""' when domainname is set and /var/yp/binding exists.
This can lead to inconsistent behavior at startup since /var may not be
mounted yet and /etc/rc's start_daemon() will not start ypbind.
A.K.A. make ypbind startup consistent with all other OpenBSD daemons.
ok deraadt@