Also generallize the yp_dbwrite functions a little: allow the caller
to specify certain flags. I need this mostly for some changes to
rpc.yppasswdd to allow in-place updates.

Also change Makefile a little to use the same format as ypserv.

R_NOOVERWRITE flag and process return codes so that we can tell the
difference between a failure due to a duplicate database entry and
failure due to some other error.

- Fix a SEGV condition in ypxfr_main.c that reared its ugly head while I
  was working on the 'parallel jobs' feature of the new yppush. After we've
  completed the map transfer and created a local temporary copy, we check
  the order number of the map on ypserv again to make sure it didn't change
  while the transfer was in progress (map skew). If for some reason we flat
  out fail to get the order number from the server, we flag this as an
  error and bail, telling ypxfr_exit() to clean up our temporary files
  for us. However, ypxfr_exit() tries to close the database before unkining
  it, not realizing that it has already been closed prior to the skew check.
  The second attempt to close the database causes a SEGV somewhere inside
  the DB code.

  (Well, it does on my 2.0.5 machine anyway. I haven't seen anyone modify
  the DB library code in ages, so the condition is probably still there.)

  To work around this, we deliberately set dbp to NULL after closing the
  database and check for the condition in ypxfr_exit(), being careful to
  avoid the second close if we see the NULL.

- In yp_dbwrite.c, make yp_open_db_rw() open the database with O_EXLOCK
  flag set. This probably won't affect much of anything, but I feel better
  having it there.

to the old one, except that it supports an additional option (-p path)
that lets you specify the top level path wiere your NIS maps live.
(ypserv allows you to specify a path like this, so it makes sense that
ypxfr should too. ypserv will automagically pass the -p flag to ypxfr
if you use a path other than /var/yp when you start it.)

This program uses client stub code generated by rpcgen as well as
the yp_dblookup.c module from ypserv.