Optional callbacks have been added to tactics that need to introduce

Optional callbacks have been added to tactics that need to introduce
new fresh names directly (i.e. without calling other tactics that need
fresh names). The default behaviour is _MUCH_ nicer than the previous
dummy one and is fully functional.

Corollary 1: all the code of tactics.cma is now reentrant.
Corollary 2: the user can be asked the fresh name if it is requested
             by a top-level tactic.