Simplify move legality check for uncommon cases

Remove a bunch of difficult and tricky code to test
legality of castle and ep moves and instead use a slower
but simpler check against the list of generated legal moves.

Because these moves are very rare the performance impact
is small but code semplification is ver big: almost 100 lines
of difficult code removed !

No functionality change. No performance change (strangely enough
there is no even minimal performance regression in pgo builds but
instead a slightly and unexpected increase).

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/book.cpp

@@ -430,7 +430,7 @@ Move Book::get_move(const Position& pos) {
       return MOVE_NONE;
 
   MoveStack mlist[256];
-  MoveStack* last = generate_legal_moves(pos, mlist);
+  MoveStack* last = generate_moves(pos, mlist);
   for (MoveStack* cur = mlist; cur != last; cur++)
       if ((int(cur->move) & 07777) == bookMove)
           return cur->move;