module installation from xml: mkdir does not like trailing slashes...

[titob]

...  when safe_mode is enabled, see also: http://bugs.php.net/bug.php?id=42739

In my opinion, that's why many people (including myself) can't install new modules from xml files, neither uploading from their browser nor downloading from the repository (it's not a permission issue, my permissions are "777")

The attached patch removes all trailing slashes from $filename when $moduledetails['isdir']

I've tested it in a pre-production environment, module installation works fine now.