ntpd: be less eager to use shorter poll intervals

* on step, poll interval drops to 8.5 mins instead of 32 seconds
* on total loss of all replies (no replies from any peer
  for last 8 requests), also drop poll interval to 8.5 mins
  instead of 32 seconds
* on send abd recv errors, RETRY_INTERVAL is now 32 seconds,
  not 5 seconds
* on timing out listening to reply, instead of unconditional
  shortening poll interval by x4, clamp it to NOREPLY_INTERVAL
  (512 seconds)
* if a largish offset is seen, clamp nexp poll interval
  to 128 seconds, not 64 seconds

function                                             old     new   delta
clamp_pollexp_and_set_MAXSTRAT                         -      37     +37
recv_and_process_peer_pkt                            861     869      +8
poll_interval                                         52      48      -4
update_local_clock                                   762     752     -10
ntpd_main                                           1063    1050     -13
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(add/remove: 1/0 grow/shrink: 1/3 up/down: 45/-27)             Total: 18 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>