A few small tweaks related to ypxfr:

A few small tweaks related to ypxfr:

- Add a ypxfr_callback() function that we can use to signal failure to
  yppush(8) in the event that we can't fork()/exec() ypxfr(8). yppush
  only checks the return status from YPPROC_XFR enough to determine
  that the RPC succeded: it relies on its callback service to figure
  out whether or not the transfer actually worked.

- Give yp_dblookup.c its own debug variable (ypdb_debug) so that DB
  access debugging messages can be turned on or off independent of the
  program's global debug messages.

- Have the Makefile rpcgen the ypushresp_xfr_1() client stub for us and
  nuke the unneeded rule for yp_xdr.c that I left in by mistake (the XDR
  filters live in libc now).