Adjust sig help implementation to match the new simpler sig help API.

This could be made simpler by computing this data (argIndex and argCount) as we go through the process of finding the argument list given our position.  However, that was not the concern of this change.  What i wanted to do here was remove the multiple entrypoints for SigHelp and move us to teh single entrypoint that matches the host side of things.

If anyone wants to go and clean up sig help now that there is no longer multiple entrypoints, feel free :)