The global nScriptCheckThreads int is confusing and is only needed for
its int-ness in AppInitMain. Move all `-par` parsing logic there and
replace the int nScriptCheckThreads ~~with a bool g_parallel_script_checks.~~ (see D521)
Also tidy up logic and improve comments.
https://github.com/bitcoin/bitcoin/pull/17342/commits/5506ecfe7a65d5705616bc048f2f1735b89993fb
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Concludes backport of Core PR17342
Depends on D7274