is to add HMACs to events to ensure they that events cannot be
injected into an experiment by an unauthorized client.

* The frontend now generates a secret key for each experiment and
  stores that into a file and in the DB.

* Each of the event clients, as well as the event producers
  (scheduler, tevc) have a new -k option to specify the name of the
  file. Two new event library functions were added for clients to give
  the key:

    event_handle_t
    event_register_withkeyfile(char *name, int threaded, char *keyfile);

    event_handle_t
    event_register_withkeydata(char *name, int threaded,
	   		       unsigned char *keydata, int keylen);

* When the library is in possesion of a key, it will generate an HMAC
  and attach it to outgoing notifications. A client receiving a
  notification will compute an HMAC and compare it against the HMAC in
  the notification. If they do not compare, the notification is
  dropped with a warning...