mysql setup: if dbuser exists try a different one (owncloud/core#108)

Fixing the oldest bug available :)
This seams to be a proper solution for owncloud/core#108.

If dbuser has no admin rights, use it.
If dbuser has admin rights and oc_[admin] does not exist, create oc_[admin].
If dbuser has admin rights and oc_[admin] exists try oc_[admin]1, oc_[admin]2, ...

What do you think?
@th3fallen this was already noted by @LukasReschke. So to quote myself:

> The thing is, that I don't want to mess around with that, as it needs to be replaced anyway. We use mysql methods there, which are outdated with PHP 5.4. But I would put that in a separate pull request.

We should open a separate issue for that.