xdr: Fix xdr_rpc* defines.

The defines for xdr_rpc* in xdr.h are wrong. It could be
very well that Solaris did strip the '_t' from xdr_u_int32_t,
but Solaris has a xdr_u_int32 function, we don't have this.
So all of this defines will lead to an unresolved symbol.

This explains why we do not use these functions in FreeBSD
while they are used in Illumos/Solaris.

Obtained from: linux libtirpc (git 7864122e61ffe4db1aa8ace89117358a1e3a391b)
MFC after: 3 weeks