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