BUG: fixes "port" DriverOption silently ignored for "mysql" driver

All DriveroOptions explicitly defined as "numeric" in libdbi (see http://libdbi-drivers.sourceforge.net/docs.html are ignored by the dbi plugin, because the wrong function is being called(dbi_conn_set_option() instead of dbi_conn_set_option_numeric()).
As a consequence, specifying a non-standard port when using the mysql driver doesn't work as expected, as it's being silently ignored.
This patch only fixes the "port" DriverOption for "mysql" in an inelegant way. A proper fix is needed, and the struct cdbi_driver_option_s defines the value as a "char". I guess it should have an additional key which is 1 if numeric, then call the proper function according to this flag.