Fix GH-9008: mb_detect_encoding(): wrong results with null $encodings

Passing `null` to `$encodings` is supposed to behave like passing the
result of `mb_detect_order()`. Therefore, we

Fix GH-9008: mb_detect_encoding(): wrong results with null $encodings

Passing `null` to `$encodings` is supposed to behave like passing the
result of `mb_detect_order()`. Therefore, we need to remove the non-
encodings from the `elist` in this case as well. Thus, we duplicate
the global `elist`, so we can modify it.

Closes GH-9063.

show more ...