Always use phpseclib for cookie encryption

- it provides fallback in case mcrypt is not found
- we now use AES in both mcrypt and PHP code case
- cleanup the code by removing mcrypt conditials
- rename some methods and cookies so that they don't refer to
  implementation details
- switching encryption implementations no longer invalidates the
  credentials

Signed-off-by: Michal Čihař <michal@cihar.com>