This patch introduces two caches using the new infrastucture, and the
concept of a 'domain'.

A 'domain' refers to a collection of clients that all have the same
view of the nfs server, and all have the same access rights (modulo
different users on the clients).  For AUTH_UNIX (and AUTH_NULL), the
domain is determined from the IP address.  For other authentication
styles, the domain might be determined directly from the credentials.

Each auth flavour knows how to allocate and free it's domain-specific
infomation.

auth_domain_cache maps a name to a domain which is owned by
an auth flavour.

ip_map_cache is a cache specific to AUTH_UNIX which maps
IP address to domain.

With this patch, svcauth_unix.c is created to store all
auth_unix related code.

The IP address lookup code is removed from nfsd/exports.c

sunrpc module initilisation is moved out of stats.c into sunrpc_syms
which seemed to be the most central .c file.  It now registers these
two caches.

Now that the caches are being used, nfsd needs to call cache_clean
periodically.