Add an optional display limit for SQL command

Hi, this is a proposition to solve the problem describe here : https://sourceforge.net/p/adminer/bugs-and-features/406/

This PR add an optional "Display limit" in the SQL Command form. When executing queries with more rows than the "Display limit", the output will be truncated. This makes it possible to export large result sets without having to display it all in the browser.

Please tell me if this solution is good for you or if you have any suggestion