Some rx type cleanup for signedness

The epoch, Cid, and security header/trailer sizes are all fundamentally
unsigned quantities.  Change the types exposed in some API signatures
to match this reality, and also change the global variables for the
epoch and Cid to match.  (Per-connection variables were already of
an unsigned type.)

Change-Id: I4a56736ef7d78028d1d0b980cda0b4c37d694388
Reviewed-on: http://gerrit.openafs.org/11106
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: D Brashear <shadow@your-file-system.com>