Added "useful" program error return codes `PROG_RET_` in "project.h". These can be used to specify which operation failed and on which input.

(Success is still main()->return 0;/exit(0); though.)

Fortune for naka's current commit: Future blessing − 末吉

include/project.h

@@ -37,5 +37,7 @@
 #define PROG_RET_UNMAP_HAYSTACK_N_FAILED(h) (((h) << 2) | 1)
 // Unmapping of argv[1] failed
 #define PROG_RET_UNMAP_NEEDLE_FAILED 3
+// Internal error
+#define PROG_RET_INTERNAL -1
 
 #endif /* _PROJECT_H */