[ticket/13138] Do not use cookie data and autologin while forcing a user_id

PHPBB3-13138
[ticket/13168] Fix mbstring warnings in ACP for PHP 5.6 compatibility

PHPBB3-13168
Merge pull request #3039 from nickvergessen/ticket/13138

[ticket/13138] Only use cookie data when we do not force a user_id
Merge pull request #3047 from Elsensee/ticket/13168

[ticket/13168] Fix mbstring warnings in ACP for PHP 5.6 compatibility

* Elsensee/ticket/13168:
  [ticket/13168] Fix mbstring warnings in ACP for PHP 5.6 compatibility