[AFS3-std] rxgk key version number of value zero

[Benjamin Kaduk]

We say that the connection's key number starts at zero, and is stored in 
the checksum field of the rx header.

I was rereading kolya's rx-spec, and noticed that "a zero checksum field 
value means that checksums are not being computed".

Given that we also say that this field is a spare field of the rx header, 
I suspect that we don't care, but might as well double-check.  (It seems 
like it would not be hard to start at 1 and skip multiples of 2^16 in the 
32-bit space.)

-Ben