Move epoch and cid generation into the rx core

Now that we have hcrypto available everywhere, we can get real randomness
in the rx core (both uerspace and kernel), and thus can initialize the
RX epoch to a real random value when first initializing a host; there is
no need to rely on rxkad to produce randomness for this purpose.
Initialize a connection ID counter at the same time, and use that in
rx_NewConnection, also supplanting rxkad-specific logic.

The rxkad-specific logic is removed, and in particular there is no longer
a need to export rxkad_EpochWasSet to the rest of the world.
The code in afs_Daemon() to check whether the rxepoch was set can be
removed, as if the epoch is not set, rx initialization fails.

Add libafshcrypto to LIBS in the handful of places it hadn't crept already.

Change-Id: I1dd1015b29a600aaf72b6b4b36f8a17032453c97
Garbage-collect afs_GCUserData's argument

We no longer need the ability to force all rxnull connections to be
reaped, as the epoch is set globally.  Change the prototype and
callers accordingly.

Change-Id: I0815fdd035c3dd9d56f72e43603f9c53f5cec79d
Remove rx_SetEpoch, rx_SetConnectionEpoch, rx_SetConnectionId

The core RX code now manages the Epoch and CID; external callers
should not be modifying them.

Change-Id: Ia517205aa280b2a0bbd2568274be7fb010fba0bc