Make afs_usrops.h more closely resemble reality

Remove prototypes for many routines which are not implemented.
(I thought some toolchains would complain about this sort of thing?
Maybe we disable it.)

Change-Id: Id09f494f1c64c2feb05ae82ead9898c08888a5de
Reviewed-on: http://gerrit.openafs.org/11547
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: D Brashear <shadow@your-file-system.com>