Replace isc_log_create/destroy with isc_logconfig_get()

Replace isc_log_create/destroy with isc_logconfig_get()

Add isc_logconfig_get() function to get the current logconfig and use
the getter to replace most of the little dancing around setting up
logging in the tools. Thus:

    isc_log_create(mctx, &lctx, &logconfig);
    isc_log_setcontext(lctx);
    dns_log_setcontext(lctx);
    ...
    ...use lcfg...
    ...
    isc_log_destroy();

is now only:

    logconfig = isc_logconfig_get(lctx);
    ...use lcfg...

For thread-safety, isc_logconfig_get() should be surrounded by RCU read
lock, but since we never use isc_logconfig_get() in threaded context,
the only place where it is actually used (but not really needed) is
named_log_init().