Integer types cleanup

Try to be a bit more strict about integer types, use off_t or int64_t for
file size and file offsets.

Replace safe_ato*() by safe_atoi32() and safe_atoi64(), fix integer types
at call sites to match.

src/main.c

@@ -475,7 +475,7 @@ main(int argc, char **argv)
             break;
 
 	  case 'd':
-	    ret = safe_atoi(optarg, &option);
+	    ret = safe_atoi32(optarg, &option);
 	    if (ret < 0)
 	      fprintf(stderr, "Error: loglevel must be an integer in '-d %s'\n", optarg);
 	    else