Document Threat[] indices
From Marco's repo.

No functional change.
Correct bench timing

No functional change.
Ensure printing UCI info in multi-pv case

After commit 94b1bbb68be6b0bc3aaf1cb804841a022bcc7007, in case available root moves are less than multiPV, we
could never reach condition:

PVIdx + 1 == multiPV

and as a consequence UCI output is not printed.

Fixed suggested by Joerg Oster.

No functional change.
Retire divide command
The main purpose of perft is to help debugging. But without the breakdown in sum of perft(N-1), it is a
completely useless debugging tool.

So perft now displays the breakdown, and divide is therefore removed.

No functional change.