Cleanup MAX_PLY

This area has become obscure and tricky over the course of incremental
changes that did not respect the original's consistency and clarity. Now,
it's not clear why we use MAX_PLY = 120, or why we use MAX_PLY+6, among
other things.

This patch does the following:

* ID loop: depth ranges from 1 to MAX_PLY-1, and due to TT constraint (depth
must fit into an int8_t), MAX_PLY should be 128.

* stack[]: plies now range from 0 to MAX_PLY-1, hence stack[MAX_PLY+4],
because of the extra 2+2 padding elements (for ss-2 and ss+2). Document this
better, while we're at it.

* Enforce 0 <= ply < MAX_PLY:
  - stop condition is now ss->ply >= MAX_PLY and not ss->ply > MAX_PLY.
  - assert(ss->ply < MAX_PLY), before using ss+1 and ss+2.
  - as a result, we don't need the artificial MAX_PLY+6 range. Instead we
  can use MAX_PLY+4 and it's clear why (for ss-2 and ss+2).

* fix: extract_pv_from_tt() and insert_pv_in_tt() had no reason to use
MAX_PLY_PLUS_6, because the array is indexed by plies, so the range of
available plies applies (0..MAX_PLY before, and now 0..MAX_PLY-1).

Tested with debug compile, using MAX_PLY=16, and running bench at depth 17,
using 1 and 7 threads. No assert() fired. Feel free to submit to more severe
crash-tests, if you can think of any.

No functional change.