eval.c: Add new var `lisp-eval-depth-reserve`

Rather than blindly increase `max-lisp-eval-depth` when entering the
debugger or running `signal-hook-function`, use this new "reserve"
to keep track of how much we have grown the stack for "debugger"
purposes so that for example recursive calls to `signal-hook-function`
can't eat up the whole C stack.

* src/eval.c (max_ensure_room): Rewrite.
(restore_stack_limits): Move before `max_ensure_room`.  Rewrite.
(call_debugger, signal_or_quit): Adjust calls accordingly.
Also grow `max-lisp-eval-depth` for `hander-bind` handlers.
(init_eval_once): Don't initialize `max_lisp_eval_depth` here.
(syms_of_eval): Initialize it here instead.
Add new var `lisp-eval-depth-reserve`.

* doc/lispref/eval.texi (Eval): Add `lisp-eval-depth-reserve`.

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