Change code for testing acceptable evasion drops

Change code for testing acceptable evasion drops

The code deciding whether a piece could be dropped to block a distant check
was dependant on the exact numbering of the variants, making it difficult
to add new variants. It is replaced by code that only requires that variants
without dropping restriction in the last two ranks come behind all others.
The number of the first such variant is given by the constant KNIGHTLESS
(which must be adapted if variants with Knights are added!).