#2 common lib should return IMA_ERROR_NOT_SUPPORTED

[Dave Wysochanski]

If all plugins don't support a given IMA function, but
return
a NULL pointer, the common library should return
IMA_ERROR_NOT_SUPPORTED. Right now, the code doesn't
handle this condition.

If we mandate all plugins provide an entry point for
all functions, this wouldn't be an issue, and the way
the common library is currently written would suffice.