Allow a slave to 'late join' another splitpoint

Instead of waiting to be allocated, actively search
for another split point to join when finishes its
search. Also modify split conditions.

This patch has been tested with 7 threads SMP and
passed both STC:

LLR: 2.97 (-2.94,2.94) [-1.50,4.50]
Total: 2885 W: 519 L: 410 D: 1956

And a reduced-LTC at  25+0.05
LLR: 2.95 (-2.94,2.94) [0.00,6.00]
Total: 4401 W: 684 L: 566 D: 3151

Was then retested against regression in 3 thread case
at standard LTC of  60+0.05:

LLR: 2.96 (-2.94,2.94) [-4.00,0.00]
Total: 40809 W: 5446 L: 5406 D: 29957

bench: 8802105
Reformat and simplify previous patch

No functional change.
Clarify some comments in SMP code

Spotted by Joona.

No functional change.
Pass Position as const ref in update_stats()

No functional change.
Make imbalance table more clear

No functional change.
Retire "Idle Threads Sleep" UCI option

After last Joona's patch there is no measurable
difference between the option set or unset.

Tested by Andreas Strangmüller with 16 threads
on his Dual Opteron 6376.

After 5000 games at 15+0.05 the result is:

1 Stockfish_14050822_T16_on   : 3003  5000 (+849,=3396,-755), 50.9 %
2 Stockfish_14050822_T16_off  : 2997  5000 (+755,=3396,-849), 49.1 %

bench: 880215
Save stalemates in TT

When there aren't legal moves after
a search, instead of returning imediately,
save bestValue in TT as in the usual case.

There is really no reason to special case
this one.

With this patch is fully fixed (again) follwing
position:

    7k/6p1/6B1/5K1P/8/8/8/8 w - - 0 1

Also in SMP case.

bench: 8802105
Remove undefended minors

Tested in "no regression" mode.

Passed both STC
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 68026 W: 12277 L: 12236 D: 43513

And LTC
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 85682 W: 12861 L: 12836 D: 59985

bench: 7311935
Drop to qsearch at low depth in razoring

If razoring conditions are satisfied and
depth is low, then directly drop in qsearch.

Passed both STC
LLR: 2.98 (-2.94,2.94) [-1.50,4.50]
Total: 12914 W: 2345 L: 2208 D: 8361

And LTC
LLR: 2.95 (-2.94,2.94) [0.00,6.00]
Total: 50600 W: 7548 L: 7230 D: 35822

bench: 8739659
Extract a reliable PV line

Truncate the extracted PV from the point where
the score stored in hash starts to deviate from
the root score.

Idea from Ronald de Man.

bench: 8732553
Fix an off-by-one bug in extract_pv_from_tt

At root we start counting plies from 1,
instead pv[] array starts from 0. So
the variable 'ply' we use in extract_pv_from_tt
to index pv[] is misnamed, indeed it is
not the real ply, but ply-1.

The fix is to leave ply name in extract_pv_from_tt
but assign it the correct start value and
consequentely change all the references to pv[].
Instead in insert_pv_in_tt it's simpler to rename
the misnamed 'ply' in 'idx'.

The off-by-one bug was unhidden when trying to use
'ply' for what it should have been, for instance in
this position:

position fen 8/6R1/8/3k4/8/8/8/2K5 w - - 0 1

at depth 24 mate line is erroneusly truncated due
to value_from_tt() using the wrong ply.

Spotted by Ronald de Man.

bench: 8732553
Symmetric king safety

Retire current asymmetric king evaluation
and use a much simpler symmetric one.

As a side effect retire the infamous
'Aggressiveness' and 'Cowardice' UCI
options.

Tested in no-regression mode,

Passed both STC
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 33855 W: 5863 L: 5764 D: 22228

And LTC
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 40571 W: 5852 L: 5760 D: 28959

bench: 8321835
Update polyglot.ini after last patch

No functional change.
Add perft 'divide' command

To show perft numbers for each move. Just
use 'divide' instead of 'perft', for instance:

position startpos moves e2e4 e7e5
divide 4

Inspired by Ronald de Man.

No functional change.
Tidy up tt.h

Backport some non-functional changes
found working on 'dense TT' patch.

No functional change.
Fix a warning with Intel compiler

warning #2259: non-pointer conversion from
"int" to "int16_t={short}" may lose significant
bits.

No functional change.
Revert "Symmetric king safety"

Regression test of 40K games at 60 secs shows
this commit to be a 2-3 ELO regression.

So revert to original king safety.

bench: 8732553
Stockfish 5

Stockfish bench signature is: 8732553
Restore development version

bench: 8732553
Retire polyglot.ini

All actively maintained GUI are able
to understand UCI protocol, so there
is no more need for this obsolete file.

No functional change.