PATH TRAVERSAL VULNERABILTY

Most ftp filesystem commands are dangerously affected by path traversal.
The reason of this, is poor path filtering in 'normalize' and 'translate' 
methods of class 'abstracted_fs' (lines 1595-1625).

Tests have been conducted on pyftpdlib 0.1.1 on both windows xp and ubuntu 
7.04, revealing the same vulnerability.

Issuing following commands, user root's parent directory will erroneously 
be listed, giving access to forbidden parts of filesystem.

CWD /
LIST ..

Same problem affects commands like STOR and RETR, allowing an attacker to 
retrieve or upload arbitrary system files. This would be only limited by 
rights under which the server is running. For any reason user must be able 
to gain access to those areas.

In order to solve the problem, I've entirely rewritten both vulnerable 
methods.
I think this solution should be robust enough to avoid any path traversal 
issue.