Include played move in atari-atari blunder analysis

[arend]

The motivation of this patch is strategy5:287:

The atari-atari analysis didn't find the combination attack after White D1. This turned out to be due to a FIXME in the atari-atari blunder code: The computation of target values didn't include the played move.

This is solved by the attached patch. Two additional changes were necessary:

• Allow owl_substantial() to be called at stackp != 0.
• The safety of the recently played move had to be set to 0 in case it depends on ok. (Otherwise atari_atari will just find the ko attack again and report it as combination attack.)

The patch doesn't actually solve strategy5:287 yet, because C2 is overvalued. The breakage is minor but slightly positive:

blunder:17      FAIL T7 [R7]
nngs4:890       PASS B13 [!B18]
ninestones:220  PASS P9 [P9|P8]
Total nodes: 1684174610 3288880 12469192