multiPV: protect mated-ins and allow thread selection
This PR is a replacement for #6652 and takes into account the changes made to this part of the code since then.
In multiPV analysis master may suppress genuine proven mated-in scores, if the iteration for a secondary PV line is aborted. In very rare cases, it may also evict a proven mated-in score from the best PV line with a score from an incomplete iteration of a secondary PV line. This PR avoids both.
In addition, we allow best thread selection in multithreaded multiPV searches. This was originally disabled in #524.
Single PV searches are not affected by this patch at all.
Fixes #6642.
closes https://github.com/official-stockfish/Stockfish/pull/6728
No functional change.