Instead of doing the lookup from ipaddr to domain inside
the nfs server, (and also when lockd calls into nfsd) it is
now done at the rpc authentication level which is a more
sensible place for it.

Note that both AUTH_UNIX and AUTH_NULL do the same lookup.

So that the rpc layer knows that nfsd and lockd both uses the
name space of domains (while other hypothetical services may
not) we introduce a 'class' for each service which svc_auth combines
with the IP address when doing a lookup.