cmArchiveWrite: Consolidate multiple ways to set thread count

Merge use of SetFilterOption() into more abstract thread count
in cmArchiveWrite constructor.

libarchive defaulting of threads for threads == 0 seems to be
configuration dependent. Preemptively default thread count via
std::thread::hardware_concurrency().

Also allow negative values for the thread count in which case
the detected hardware concurrency is also used but the given
absolute thread count is used as an upper limit.