Be more explicit about our checking for database types. This prevents random fatal errors if the appropriate database extension isn't installed, because ADODB isn't smart enough to test for this itself.

Also deprecate the 'mysql' database type, in favour of 'mysql5'. This brings it into alignment with the 'postgres8' type. The 'mysql' type will issue a log_environ error in 1.0 and crash in 1.1.