zstd: Implement core detection (#2083)

zstd: Implement core detection (#2083)

The bsdtar manual page claims that setting zstd:threads to 0 tells zstd
to use as many threads as there are cores in the system, but it actually
disables multi-threading.  Replace 0 with the number of configured
processors.

While here, add a previously missing overflow check.

Co-authored-by: Martin Matuska <martin@matuska.de>