Improve cache hits

If cdUp is performed after entering into an empty directory, cache fails
even if it has entires of the parent.

This is because an empty directory is not maintained by cache. Whenever
accessing the empty directory, cache always fails.

To avoid this problem, insert an invalid empty entry at first.
Cache non-existent entries as well

This improves cache performance. Sometimes Qt passes a lowered file name
even if it has mixed cases. In this case, FtpFileEngine tries to read
the directory entires to retrive its infomations and to test it exists
really.

The problematic url is ftp://ftp.netlabs.org/incoming/Qt.
Check the success status of connect and login
Distinguish connections depending on users