Set service-specific data for rxgkd

It's not used yet.

Change-Id: Ibdcc84604f35f7a2d13b57e3b3c79ea295a26032
Export ServiceSpecific routines from libafsrpc

We will use them for rxgk's key-negotiation service.

Change-Id: Ib0b0d83ea87daa51a6798b3cb3c3d601d6416fc2
Use service-specific getkey for token-encrypting keys

Do not hardcode a function to obtain a long-term server key with
which to encrypt the token to be returned.  Instead, use the getkey
function from the service-specific data to get a key.

Change-Id: I09f1d29986d5f11942c71095765ddf73cb17f78a
Add rxgk_NewService_SecObj

To call rxgk_NewServerSecurityObject and rx_NewService for the
key-negotiation service, and register the service-specific data
for that service at the same time.

We will need an rxgk_NewServiceHost_SecObj eventually as well, but
that's more code and this is mostly still a prototype.

Change-Id: Ib21ae54a56f0da919e7e9f94c9709e8b1dd66761
Use rxgk_NewService_SecObj in rxgkd

Simplify things down a bit.

Change-Id: Ib92bd76bfa81e28bc65f50b3452618401341c088
rxgk_NewEphemeralService_SecObj

Generate a random key, and generate rxgk security object and
rx service (with service-specific data) which will use that
key as their long-term key.

Change-Id: I164b6ede486239e84f9adea9a08adf87cc8f02db
Use rxgk_NewEphemeralService_SecObj for rxgkd

An ephemeral key is fine, here.

Change-Id: Iafe0ec0aeb08a189820d55593eef832672402b79