An rpc service like NFSD knows about a number of
versions (2,3,4) and needs to know the max and min valid
version numbers, and also the maximum xdr buffer size used
by any version.

These max/mins are currently computed at compile time which means
we need to include all the xdr*.h headers into nfssvc.c just for
computing a couple of numbers.

With this patch, these max/mins are calculated at run time from
information in the svc_version structure which has been extended to
include per-version xdrsize.

Also the nfsd_version2, nfsd_version3, and nfsd_version4
structures are moved from nfssvc.c to their more correct home
of nfs*proc.c

Code gets neated, xdr*.h are no-longer included in
nfssvc.c, and the ugly ifdef can be removed fom xdr4.h