[RFA] Avoid recursivly defined user functions.

[Paul Hilfinger]

> How about something even simpler - track user command depth at runtime? 
> Set an absurd limit, like 1024 deep, if we can handle that in a
> normal-sized stack limit.  Then complain if we hit it at runtime.

I completely agree with this.  After all, if you were willing to
contemplate outlawing recursion altogether, you certainly won't LOSE
anything by Daniel's approach, and you gain everything you wanted in
the first place---to avoid crashing GDB.  

Paul