Fix go mate x in multithreading

[peregrineshahin]

Fixes two issues with master for go mate x:

• when running go mate x in losing positions, master always goes to the maximal depth, arguably against what the UCI protocol demands
• when running go mate x in winning positions with multiple threads, master may return non-mate scores from the search (this issue is present in stockfish since at least sf16) The issues are fixed by (a) also checking if score is mate -x and by (b) only letting mainthread stop the search for go mate x commands, and by not looking for a best thread but using mainthread as per the default.

Related: niklasf/python-chess#1070

More diagnostics can be found here https://github.com/peregrineshahin/Stockfish/pull/6#issuecomment-1983100667

No functional change.

[peregrineshahin]

@robertnurnberg I just found out that this effectively reverts this commit, but other the fixes are not trivial...
d5f883a

[robertnurnberg]

Noted. I think it is best to merge this PR as an urgent fix as it is.

Improving mate finding for go mate x commands in multithreading mode can then be left for the future. It won't be easy to generalize @vondele 's old patch to the new best thread selection algorithm.

[robertnurnberg]

Just for completeness, running the testing script from https://github.com/peregrineshahin/Stockfish/pull/6#issuecomment-1983100667 with more threads and up to go mate 16 positions also did not result in any errors for this patch:

Running ./stockfish with 8 threads and 1024MB hash, up to 'go mate 16'.
run    10: error count = 0, short count = 0, maxdepth count = 0.
<snip>
run  1990: error count = 0, short count = 0, maxdepth count = 0.
run  2000: error count = 0, short count = 0, maxdepth count = 0.