[ticket/10605] Check for orphan privmsgs when deleting a user

Also moved the hole code into a new function.

PHPBB3-10605
[ticket/9089] Add tabindex to PM recipient box, to allow tabbing to the subject

I also added tabindex 1 to the buttons for adding the recipients.
Also note, that duplicated tabindex are fine:
http://www.w3.org/TR/html4/interact/forms.html#adef-tabindex

PHPBB3-9089
[ticket/10605] Delete orphan private messages on update

PHPBB3-10605
[ticket/9089] Add tabindex to pm/topic/post icon-options aswell

PHPBB3-9089
[ticket/10605] Prefix function with phpbb_ and use true instead of 1

PHPBB3-10605
[ticket/8743] Include poster´s name in mail notifications

PHPBB3-8743
[ticket/10605] Use unset() instead of checking user_id over and over again.

PHPBB3-10605
[ticket/10605] Turn $undelivered_user into a real array of counters.

PHPBB3-10605
[ticket/10605] Remove unused variable declarations.

PHPBB3-10605
[ticket/10605] Remove unnecessary array_keys calls on $delete_rows.

PHPBB3-10605
[ticket/10605] Remove unnecessary $delete_ids array.

PHPBB3-10605
[ticket/10605] Break long comment into multiple lines 80 chars short.

PHPBB3-10605
[ticket/10605] Rename $delete_rows to $delete_ids.

PHPBB3-10605
[ticket/10605] Split query to be able to use indexes

PHPBB3-10605
[ticket/10605] Reset user´s pm count to 0 when deleting his PMs

PHPBB3-10605
[ticket/10561] All users can choose deactivated styles (fixed).

A form exploit enabled the users to select a deactivated
style. Fixed with extra check on submit, with a new function
styles_verify to check if the selected style is activated or not.

PHPBB3-10561
[ticket/10561] Changes made to function phpbb_style_is_active().

Fixed return type, documented function and, removed
style_id from fetch.

PHPBB3-10561
[ticket/10561] Changes made to $db->sql_fetchrow().

Changed $db->sql_fetchrow() to $db->sql_fetchfield().

PHPBB3-10561
[ticket/10561] Fixed syntax error and renamed return variables.

Renamed $style_verified to $style_is_active and fixed extra ';'.

PHPBB3-10561
[ticket/10561] Moved and renamed the funtion validate_style().

Fixed minor changes as suggested by @bantu.

PHPBB3-10561