Fixes to C++ generic IDL port generation

[jirwin-geon]

Resolves several issues with custom IDL interfaces and C++ ports:

• Nested structs are resolved (previously caused redhawk-codegen to crash).
• Correct determination of fixed-length vs. variable length structs in argument signatures (previously generated code that did not compile).
• Better resolution of typedef-ed types.
• Proper handling of union types (same rules as structs).

Testing requires installing custom IDL project to $OSSIEHOME. An example IDL project plus test components is attached: IdlArgs.zip.