Fix guard condition for nested cofixpoints.

There were actually two problems, one of them being clearly unsound.
To make sure that this does not show up somewhere else in the code, it would
be better to resort to an abstraction keeping in sync the environment and the
De Bruijn index of the current cofixpoints, like guard_env does for fixpoints.

Conflicts:
	kernel/inductive.ml
Fix guard condition for nested cofixpoints in checker.

Conflicts:
	checker/inductive.ml