[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 page type is using the `{SCRIPT_NAME}` variable, which is unreliable.

PHPBB3-12364