Now removing stray old cookies from 5.0.12

Cookies from 5.0.12 seemed to have an extra slash in the path.
Firefox doesn't allow to remove them if the trailing slash isn't there,
thus making it impossible to logout correctly.

This fix adds extra code to delete such stray cookies.
Merge pull request #5723 from owncloud/stable5-strayremembermecookiesfix

Now removing stray old cookies from 5.0.12