Change default from mysql to mysqli

The mysql extension is deprecated as of PHP 5.5.0 [1] so we use mysqli
as our default database type.

Commit includes relevant changes in installer, admin checks and
documentation (admin guide).

Fixes #16941

[1] http://php.net/intro.mysql
db_prepare_string() remove special handling for mysql

Since ADOdb's qstr() method calls mysql_real_escape_string() /
mysql_real_escape_string() depending on driver type, it does not make
sense to have special logic within database api to handle mysql
differently from mysqli. Let ADOdb quote the string.