Warning cleanups for kernel module build

Warning cleanups for kernel module build

Take care of various warnings (about 70) during a libafs build.
A non-exhaustive list of changes:
- Unused variables, in some cases to be IFDEFed along with the code
that uses it
- Cast some const char * to char * for the afs_* functions
- Move afs_UFSCacheFetchProc and afs_UFSCacheStoreProc declarations
to afs_prototypes.h
- Include afsutil.h in error_msg.c to get strlcpy
- Change type of argument 3 in export_sysname, to match what the callers
are actually doing
- Rework a few lines in pthread_glock.c to avoid a warning about unused
value
- Fix a warning in rpc_scan.c that really looks like a bug (but
probably harmless in our case)

Reviewed-on: http://gerrit.openafs.org/http://gerrit.openafs.org/151
Reviewed-by: Russ Allbery <rra@stanford.edu>
Tested-by: Russ Allbery <rra@stanford.edu>