tweaks: rename two variables and improve two comments

src/utils.c

@@ -108,19 +108,19 @@ int digits(ssize_t n)
 }
 #endif
 
-/* Read an integer from str.  If it parses okay, store it in *result
- * and return TRUE; otherwise, return FALSE. */
-bool parse_num(const char *str, ssize_t *result)
+/* Read an integer from the given string.  If it parses okay,
+ * store it in *result and return TRUE; otherwise, return FALSE. */
+bool parse_num(const char *string, ssize_t *result)
 {
-	char *first_error;
 	ssize_t value;
+	char *excess;
 
-	/* The manual page for strtol() says this is required. */
+	/* Clear the error number so that we can check it afterward. */
 	errno = 0;
 
-	value = (ssize_t)strtol(str, &first_error, 10);
+	value = (ssize_t)strtol(string, &excess, 10);
 
-	if (errno == ERANGE || *str == '\0' || *first_error != '\0')
+	if (errno == ERANGE || *string == '\0' || *excess != '\0')
 		return FALSE;
 
 	*result = value;