Connect to the database server identified by aDSN using the given
username and password. The DSN is in the format
dbi:DriverName:dbname=database_name;host=hostname;port=port
Where dbi is constant, DriverName is the name of the driver, and
everything else is parameters in the form name1=value1;name2=value2;...
Individual drivers may parse the parameters differently, though
the existing ones all support parameters dbname, host and port.