fulltext_native.php on innodb loading deadly slow for big indexes

when using INNODB and having a BIG search index (like 25 million entries) on heavily loaded database, opening the maintenance->search index page is deadly slow (like 5+ minutes to load).

After tracing the issue, I notices phpBB is issuing a SELECT count on that tables, which causes a full table scan on innoDB.
It would be better to use SHOW TABLE STATUS here to obtain the row count.

I'll attack a patch that uses show table status instead count when using MySQL.