Database: Fall back from ext/mysqli to ext/mysql if the connection fails.

This allows us to avoid breaking a site that works under ext/mysql but is misconfigured for ext/mysqli.

props pento.
see #21663.


git-svn-id: https://develop.svn.wordpress.org/trunk@27935 602fd350-edb4-49c9-b593-d223f7449a82