sq_attackers() + others (see dedails). Ready for move do/undo ?

- add many "const" in func parameters
- attack.c: sq_attackers()
- move print_board_raw from position.c to to board.c
- move some fen_check() tests to pos_check()
- add REL_RANK() macro. TODO: add one more for bitboards
- fen.c: more tests for FEN validity
- position.c: add pos_checkers() and pos_check()
- tests: add common-test.h (for shared FEN positions access)

src/fen.h

@@ -14,7 +14,7 @@
 #ifndef FEN_H
 #define FEN_H
 
-#include "position.h"
+#include "chessdefs.h"
 
 #define FENSTRLEN 92                              /* secure FEN string size */