Suggestion: Writable files warning

[AndyF]

Along the same lines as we have a warn box in the ACP's home page for the installer being present, I think adding a warning if conf_global is found to be writeable would be a good idea too. :)

I don't think there is a need to duplicate all warnings from the Security Center into the home page but: As this file is typically going to have its permissions altered for an upgrade, a warning box in the ACP home page would be good to prevent any chance of it being left in that state.

I guess this will probably only work on a 'nix box though anyway. :)