Don't pass NULL to the RPC function

It tries to encode it, which doesn't work so well.
Pass an empty (zero-length) token instead, per the spec.

Change-Id: I6c8a7e7931a841bf81003e0e276eba54620c8bf8
Enforce that RXGK_AFSCombineTokens is only called on rxgk connections

Change-Id: I2b4997009b303c75da81f1a1940ee2ae4d2195e7
Make calloc return zeroed memory

As far as I can tell, the afs_osi_Alloc contract does not
guarantee zeroed memory.  On FreeBSD, with a debug kernel, it
definitely does not currently provide zeroed memory, returning
instead memory initialized with 0xdeadc0de.

Change-Id: I187c2057d473fba869692c1dfa11735502b260c1
Handle the no-cm_tok branch of RXGK_AFSCombineTokens

You know, the one that we actually will *use* in the minimum
viable product.

Change-Id: I7dd4bb471931e681a4ad1ef5b6a1f00e6185b4b3