[ticket/12993] Improve get_user_ranks

Explanation in http://area51.phpbb.com/phpBB/viewtopic.php?f=108&t=46051

PHPBB3-12993
[ticket/12993] Fix user_loader and memberlist

PHPBB3-12993
[ticket/12993] Rename to phpbb_get_user_rank

PHPBB3-12993
[ticket/12993] Fix includes of functions_display

PHPBB3-12993
[ticket/12993] Return an array instead of reference passing

PHPBB3-12993
[ticket/12993] Update doc block

PHPBB3-12993
Merge pull request #2888 from Nicofuma/ticket/12993

[ticket/12993] Improve get_user_ranks