Performace improvements/simplifications/cleanups:

- Make the child process reaper signal-driven. (Previously, we called reaper()
  once a second each time we went through the select() loop. This was
  convenient, but inefficient.)

- Increase main select() timeout from 1 second to 60 seconds and use
  this as the ping timer instead of using timestamps in the _dom_binding
  structure. This nd the reaper() change noted above makes ypbind a little
  less CPU-intensive.

- Don't flag EINTR's from select() as errors since they will happen as a
  result of incoming SIGCHLD's interrupting select().

- Prevent possible resource hogging. Currently we malloc() memory
  each time a user process asks us to establish a binding for a domain,
  but we never free it. This could lead to serious memory leakage if a
  'clever' user did something like ask ypwhich to check the bindings
  for domains 0.0.0.0.0.0.0.0.0.0 through 9.9.9.9.9.9.9.9.9.9 inclusive.
  (This would also make a mess out of the /var/yp/binding directory.)

  We now avoid this silliness by a) limiting the maximum number of
  simultaneous bindings we can manage to 200, and b) free()ing _dom_binding
  structures of secondary domains whose servers have stopped responding.
  We unlink the /var/yp/binding/domain.vers files for the free()ed
  domains too.

  (This is safe to do since a client can prod us into reestablishing the
  binding, at which time we'll simply allocate a new _dom_binding structure
  for it.)

  We keep count of the total number of domains. If asked to
  allocate more than the maximum, we return an error. I have yet to hear
  of anybody needing 200 simultaneous NIS bindings, so this should be
  enough. (I chose the number 200 arbitrarily. It can be increased if need
  be.)

- Changed "server not responding"/"server OK" messages to display server
  IP addresses again since it looks spiffier.

- Use daemon() to daemonify ourselves,

- Added a SIGTERM handler that removes all binding files and unregisters
  the ypbind service from the portmapper when a SIGTERM in received.

- The comment 'blow away everything in BINDINGDIR' has no associated code.
  Give it some: clean out /var/yp/binding at startup (if it exists).

This completes my ypbind wishlist. Barring bug fixes, I shouldn't need to
go poking around in here anymore. (Of course, this means I can start
working on my ypserv whishlist now... :)

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