Use portable format specifiers: Clarify some casted expressions.

Though correct (to my understanding), some of the casts introduced in
previous commits could lead to confusion, by casting only the first
expression component instead of the whole expression.

This fixes that, parenthesizing casted expressions, except where
operator precedence rules make it unneccesary.

src/eval.c

@@ -9965,7 +9965,7 @@ static void f_getregtype(typval_T *argvars, typval_T *rettv)
   case MCHAR: buf[0] = 'v'; break;
   case MBLOCK:
     buf[0] = Ctrl_V;
-    sprintf((char *)buf + 1, "%" PRId64, (int64_t)reglen + 1);
+    sprintf((char *)buf + 1, "%" PRId64, (int64_t)(reglen + 1));
     break;
   }
   rettv->v_type = VAR_STRING;