Always-0 NFS statistics

[Alan Somers]

It looks to me like three of the server statistics published by nfsstat are
always 0.  At least, cscope can't find any location in the kernel that sets
these variables.  Are they legitimately always zero in FreeBSD's
implementation of NFS, or are they reserved for future features, or is this
a mistake?  The stats are:

nfsstat's name    kernel name
Retfailed         srv_errs
Faults            srvrpc_errs
Idem              srvcache_idemdonehits