mxstartup.service: Add explicit network.target dependency

Currently mxstartup.service has a After dependency on nis.service and
nis.service has a After dependency on network.target, so we have an
implied After dependency on network.target.

We need to have the network to start mxstartup services.

We are going to disable and remove nis.service, so we can no longer
depend on the implied dependency.

Add After dependency on network.target.
Remove dependency on nis.service.

mxstartup.service

@@ -1,6 +1,6 @@
 [Unit]
 Description=Mariux mxstartup classic
-After=mxmount.service nis.service
+After=mxmount.service network.target
 Requires=mxmount.service
 
 [Service]