Add `redisplay_counter` to catch nested redisplays and abort outer one

The redisplay code is not re-entrant.  To allow running ELisp code
from within redisplay, we have some hacks (e.g. `inhibit-redisplay`)
that try to avoid the resulting breakage.
This commit adds another one of those hacks, which tries
to get closer to the core of the problem, thereby making it "safe"
to override `inhibit-redisplay`, e.g. to debug jit-lock code.

* src/dispextern.h (redisplay_counter): Declare.
* src/xdisp.c (redisplay_counter): Define.
(redisplay_internal) Increment it.
(dsafe__call): Use it, in case `inhibit-redisplay` is overridden.
* src/eval.c (call_debugger): Use it as well to refine the test
we already had.

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