Check for an available slave early on

Don't take the split lock if we don't have
available slaves (about 30-40% of times).

This new condition allows to retire the now
redundant one on number of threads.

No functional change.
Small renaming in Tracing

No functional change.
Add bonus for advanced lever

Passed both STC
LLR: 2.95 (-2.94,2.94) [-1.50,4.50]
Total: 15413 W: 2670 L: 2530 D: 10213

And LTC:
LLR: 2.95 (-2.94,2.94) [1.00,6.00]
Total: 66908 W: 9398 L: 8960 D: 48550

Bench : 7859385
Tweak outpost name

This name is more accurate, since that function evaluates only one
outpost in every call.
No functional change.
Simplify pawn threats and merge into ThreatenedByPawn[]

Tested in no-regression mode,

passed STC
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 14477 W: 2493 L: 2362 D: 9622

and LTC
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 51964 W: 7091 L: 7013 D: 37860

bench: 7875814
Factor out pawn attacks in 'lever'

Improves readibility and possibly speed.

No functional change.
Triviality in UCI::loop

Code style paranoid in action here :-)

No functional change.
Fix a warning with MSVC in 'analyze' mode

Here MSVC is worried that

StepAttacksBB[PAWN][psq]

could overflow, so change psq initialization
to clarify psq is always less than 64.

No functional change.
Merge remote-tracking branch 'upstream/master'