widl: Automatically add "handle_t IDL_handle" parameter to functions with no explicit handle specified whose containing interface has the explicit_handle attribute.

widl: The implicit_handle attribute is allowed with a handle explicitly specified in the function parameters.

In that case, that handle is used instead of the implicit handle.

Fix the check for the explicit_handle attribute being specified without
a handle being specified in the function parameters, even though issuing
an error is wrong. (Thanks to Marcus Meissner & Coverity for spotting
that the check didn't do what it was supposed to do.)

widl: Check that fields in structures and unions referenced by non-local functions can be marshalled and that their attributes are consistent.

widl: Stop looping in check_remoting_args when a context_handle or wire_marshal type is found as they are in effect fundamental types.