[ticket/12364] Add template identifier var to all missing pages

viewtopic.php has `S_VIEWTOPIC`
viewforum.php has `S_VIEWFORUM`
search.php has `S_IN_SEARCH`
ucp.php has `S_IN_UCP`
mcp.php has `S_IN_MCP`

It would be good to have `index.php`, `viewonline.php`, `faq.php`, `memberlist.php`, `posting.php` and `report.php` use this method too.

Why? The only other guaranteed method to get the correct page type is using the `{SCRIPT_NAME}` variable, which is unreliable.

PHPBB3-12364