mountd: clean up cache API

Clean up: Squelch compiler warnings and document public parts of
cache API.

cache.c: At top level:
cache.c:67: warning: no previous prototype for auth_unix_ip
cache.c:123: warning: no previous prototype for auth_unix_gid
cache.c:217: warning: no previous prototype for get_uuid
cache.c:247: warning: no previous prototype for uuid_by_path
cache.c:326: warning: no previous prototype for nfsd_fh
cache.c:745: warning: no previous prototype for nfsd_export
cache.c:820: warning: no previous prototype for cache_open
cache.c:832: warning: no previous prototype for cache_set_fd
cache.c:841: warning: no previous prototype for
cache_process_req
cache.c:921: warning: no previous prototype for cache_export
cache.c:953: warning: no previous prototype for
cache_get_filehandle

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>