Remove Now Unneeded Help Text

With the new automatic setting of split depth
instead of a default, the user no longer needs
guidance on setting the split point.

Also threads now defaults to one.

No functional change.
Bonus for a pawn in front of knight/bishop

Idea originated from a post of Don Dailey
on talkchess and reported by Eelco.

This is the last succesful attempt of a long
series of trials (as usually happens, the
'idea' alone is not enough).

Passed both short 15secs TC
LLR: 2.97 (-2.94,2.94)
Total: 7629 W: 1645 L: 1515 D: 4469

And long 60secs TC
LLR: 2.96 (-2.94,2.94)
Total: 10218 W: 1932 L: 1775 D: 6511

bench: 4944581
Use exceptions to stop the search

Instead of classical flags, throw an
exception when we want to immediately halt
the search. Currently only one type
is used for both UCI stop and threads
cut off.

No functional change.
Some renaming in TT store()

No functional change.
Simplify DistanceRingsBB init

Verified by same benchmark and picking some random values.

No functional change.
Revert using exceptions

Due to crashes. It will be reapplied once
we understand what's happening.

No functional change.