refclock: check all driver options

In each driver provide a list of supported options and abort when an
unknown option is specified in the refclock directive.

refclock_sock.c

@@ -101,6 +101,8 @@ static int sock_initialise(RCL_Instance instance)
   int sockfd;
   char *path;
 
+  RCL_CheckDriverOptions(instance, NULL);
+
   path = RCL_GetDriverParameter(instance);
  
   s.sun_family = AF_UNIX;