Get rid of service_type defaulting completely, and related things

Now "up" and "down" have to be defined if you want to use them,
  and there is no default service type.
Relatedly, the plugins and the core mon API support empty
  service_types (which act as if UP for result purposes), and
  these resources do not show up in monitor output lists.
A new interface was defined for more "advanced" monitor plugins
  to manage sttl_t directly, which plugin_static uses to
  allow the definition of service_types like the previous
  built-in "up" and "down"...
add TTL_MAX define for better readability