Improved permission detection when using ftp backend

[aseques]

Added the same improvements when using the ftp backend that allow better permission checking when on POSIX systems.
Basically adapted the patch from here: http://pyd.io/f/topic/solved-linux-acl-folder-not-writeable/

If applicable, copyright goes to the projec..

[cdujeu]

@aseques, the real path here is referring to something like ftp:// ... can we really check POSIX on a remote server via FTP?

[aseques]

The issue in my case is that a user (user1) connects to an ftp and has read/write access to a subfolder owned by another user (user2)

user1
---------user2
----------------- files

Unless the check is done via posix_access, the is_writable() will return false when checking permissions on user2 folder.

[cdujeu]

@c12simple what do you think?

[c12simple]

Hello @aseques
Could you please tell me how to reproduce this issue?
FTP service: the OS of FTP, ftp server program , permission setting. .....

[c12simple]

Hello @aseques
Please contact me when you have time.
THank you very much.

[aseques]

Sorry I can't remember the details, it's been a lot of time (I even changed jobs), but it basically it's in the issue. I was using proftpd with the database backend (mysql).

[c12simple]

Hello @aseques
Thank you, that's enough for me to reproduce by using proftpd.