OSLQuery: reliably tell when no constant default is available for a param.

There's always been a 'validdefault' field in the OSLQuery::Parameter
structure, but it was always set to true.

* oslc -- add a %initexpr hint in the oso for parameters that are initialized
  by expressions (like myparam = u, or myparam = noise(P), i.e. is not a
  constant value at shader compilation time.

* oslquery -- if this hint is encountered for a parameter, mark its
  valliddefault = false.

Thus, a program using OSLQuery ought to be able to check the validdefault
field, and if it's true, the default constant is valid, but if it's false,
you can't count on the default constant being the true default.