SFTP: add the ability for nlist() and rawlist() to be sorted

Also remove `$realpath` stuff from `_list()` since it's not used anymore.
```php
    public function testFileExistsIsFileIsDirFileNonexistent($sftp)
    {
        $this->assertFalse(
            $sftp->file_exists('file4.txt'),
            'Failed asserting that a nonexistent file does not exist.'
        );

        $this->assertFalse(
            $sftp->is_file('file4.txt'),
            'Failed asserting that is_file() on nonexistent file returns false.'
        );

        $this->assertFalse(
            $sftp->is_dir('file4.txt'),
            'Failed asserting that is_dir() on nonexistent file returns false.'
        );

        return $sftp;
    }

    /**
    * @depends testFileExistsIsFileIsDirFileNonexistent
    */
    public function testSortOrder($sftp)
```
In that case it seems like it's linear already? At least for this segment? `testFileExistsIsFileIsDirFileNonexistent` is the method that immediately precedes `testSortOrder`, thus linearity is preserved? Before `testFileExistsIsFileIsDirFileNonexistent` is `testFileExistsIsFileIsDirFile` and then `testChDirOnFile` so it's like 3 methods away from `testSortOrder`.