[TYPO3] Fix for changing the BE admin url

[Matthew Manderson]

> I'm still testing this, but I'd welcome your thoughts and if deemed a
> good solution the documentation and wiki page
> (http://wiki.typo3.org/index.php/Security#Change_the_TYPO3_BE_directory)
> could be updated.

I think this is a really good idea. Thanks for your effort. Both from a
client facing perspective and from a 'security' perspective. Typo3 itself
is about as secure as it can be. Anything to allow installations to be a
little more unique will help.

It would be particulary useful to hear from the dev team if this approach
you suggest is a technically a good one and ideally if it can be included
in the BE install tool.

Matthew