Check dirs are writable

[mattpass]

Add is_writable checks on dirs that need write perms (data and tmp only?) in requirements.php inform user if unable to write in those.

Right now if it can't write in those requirements.php claims there's a session problem, which is true but only because it can't write in the dir.

[mattpass]

Now have checks in place to confirm the data dir exists, is readable and writable, before the session check so the user doesn't get a misleading error. The tmp, plugins etc dirs will be moved to data dir in the future so this is enough as a check.