return an isc_logconfig_t.  isc_log_cretechannel and isc_log_usechannel
now take an isc_logconfig_t instead of an isc_log_t; this is to address
reconfiguration in a multithreaded environment.

isc_logconfig_create, isc_logconfig_get, isc_logconfig_use and
isc_logconfig_destroy were added to work with the new isc_logconfig_t type.

isc_logregister_categories and dns_log_init are now both void rather than
returning isc_result_t.