df: --output w/o "=" doesn't expect further args

"df --output ." was treated as "df --output=." and hence "." was
interpreted as a column name. With this commit, "." is treated as
an argument on its own.

Fixes #3324

tests/by-util/test_df.rs

@@ -80,6 +80,11 @@ fn test_output_option() {
     new_ucmd!().arg("--output=invalid_option").fails();
 }
 
+#[test]
+fn test_output_option_without_equals_sign() {
+    new_ucmd!().arg("--output").arg(".").succeeds();
+}
+
 #[test]
 fn test_type_option() {
     new_ucmd!().args(&["-t", "ext4", "-t", "ext3"]).succeeds();