update the rendezvous spec to be less wrong (but still wrong)

   The first time the OP provides an advertised service, it generates

   a public/private keypair (stored locally).  Periodically, the OP

   generates service descriptor, containing:

   generates a service descriptor, containing:

         KL    Key length                            [2 octets]

         PK    Bob's public key                      [KL octets]

   KL is the length of PK, in octets.  (Currently, KL must be 128.)

   TS is the number of seconds elapsed since Jan 1, 1970.

   The members of Ipt may either be EITHER nicknames, or identity key

   digests, encoded in hex, and prefixed with a '$'.  Servers must accept

   both forms.  Before 0.0.7 is obsoleted, servers should only generate the

   first form.  After, servers should only generate the second form.

   The members of Ipt may be either (a) nicknames, or (b) identity key

   digests, encoded in hex, and prefixed with a '$'.  Clients must

   accept both forms. Services must only generate the second form.

   Once 0.0.8.1 and 0.0.9pre3-4 are obsoleted, we can drop the first

   form.

   [It's ok for Bob to advertise 0 introduction points. He might want

    to do that if he previously advertised some introduction points,