rpcgen-erated on the fly (just like librpcsvc).
Makefile: Add rule for generating yp_xdr.c and yp.h.
xdryp.c: gut everything except the special ypresp_all XDR function
needed to to handle yp_all() (this one can't be created on
the fly), and xdr_datum(), which isn't used internally by
libc, but which as documented as being there in yp_prot.h,
so what the hell. We now get everything else from yp_xdr.c.
yplib.c: change a few structure member names to match those found in
yp.h instead of those declared in yp_prot.h.
