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

Also remove `$realpath` stuff from `_list()` since it's not used anymore.
If it's not supposed to be the method that immediately precedes it than it seems to me that no method should depend on one that's supposed to fail, as `testChDirOnFile` is supposed to do?

If you want to get right down to it it seems like `testSortOrder` (the function that that comment immediately precedes) depends on `testTruncate()`, `testTouch()` and `testPutSizeGetFile()`. And potentially to a lesser extent on `testChDirScratch()` since the directory `testSortOrder()` needs to be ran in needs to be clean of any other files / directories aside from those that it is expecting.