Limit the scope of valid properties to the service they apply to

[pepone]

We recently introduced language specific properties, which allows to narrow the scope of properties.

It would be good to have a similar mechanism for "service" specific properties. It doesn't make sense that all C++ applications accept "Glacier2" properties as valid.

We can have a set of "enabled" prefixes, for example Ice, IceSSL should be always enabled and validated.

Others Glacier2, IceGrid, IceStorm should be "reserved" but "disabled", if an application sets such a property its an error.

Then we need a way to enable, the additional prefixes, in which case we validate that the properties are valid.