Add NetMaskRequired option into NetworkField and NetworkValidator

vpn: split off PPTP, L2TP and PPPoE servers

This way, the caller only needs to specify plugins.inc on the
top to let the subsystem(s) know plugin hooks need to be executed.

This is done so that services.inc can load plugins when
plugins.inc was invoked, if not, it stays clear of the
extra work.

The rules are simple:

* plugins_configure() requires all available plugins, probes their
  configure functions and then calls the advertised functions.

* All GUI pages must directly include their includes in plugins.inc.d
  to not overly pollute their loading phase.