[ticket/12426] Rework of user search

Changes proposed by @nickvergessen done:
Search fields definition done in profile field type class
if(!empty($additional)) used (and the { is on a new line
Tried to avoid nesting of classes

Bug found and corrected:
memberlist_search.html uses {S_MODE_ACTION}
  should use {S_USER_SEARCH_ACTION}

TO DO:
Rework of search array definition
Get the $sql_where from field type class

PHPBB3-12426