Update admin-search.php to prevent from molesting db without reason

Currently every time user searches something, it will search everywhere without letting user choose what type of search he wants. 
After it loads all search results - user will choose search type and it will search again. 
If users won't provide keyword to search it will output everything from a DB. On very large DBs it may take hours and slow down server a lot.

It's also an easy way to kill db server.

But main reason... such code just shouldn't be there :)