Assign explicit opcodes to butc RPCs

Assign explicit opcodes to butc RPCs

This should prevent inadvertent reassignment if additional RPCs
are introduced in the future.

Change-Id: I5645ca478d2ecef9962f4bde04ab8f9895dd9497
Reviewed-on: https://gerrit.openafs.org/13317
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>