Fix a bug in the async DNS resolver that can crash ypserv. yp_prune_dnsq() is not sane: if the TTL on a pending but unanswered query hits 0 and the circular queue entry is removed and free()d, the for() loop may still try to use the entry pointer (which now points at no longer valid memory). usually, deleting only the last entry off the end of the queue worked, but if more than one was deleted, the server would crash. I changed things a bit so this shouldn't happen anymore.
Fix a bug in the async DNS resolver that can crash ypserv. yp_prune_dnsq()
is not sane: if the TTL on a pending but unanswered query hits 0 and the
circular queue entry is removed and free()d, the for() loop may still try
to use the entry pointer (which now points at no longer valid memory).
usually, deleting only the last entry off the end of the queue worked, but
if more than one was deleted, the server would crash. I changed things a
bit so this shouldn't happen anymore.

Also arranged to call the prune routine a bit more often.

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