todo

实现linux系统和网络时钟服务器的时间同步,从而可以得到准确的时间.

Ask on newsgroup about making sanity checks mandatory, default, or optional
   Rob Landley says disable checks with CONFIG to save space
   good to know why something fails
   Denis Vlasenko recommends not checking UDP source, since that can
     break on multihomed server, confirmed for ntpd
   Denis also says "Give a message. You may make it configurable:
     ntpclient: dropped malformed packet[: <optionally what's wrong>]
Ask for pre-release comments from Walter Harms and others
Exit without warning if -f is the only option

Leap second, crib from openntpd?  worthless!  Look at wikipedia "unix time" article
IPv6?
Multiple (interleaved) servers
   compile-time option for unlimited (malloc) or limited (no-malloc,
   for hard-embedded use) number of servers
Test handle laptop suspend?
Drift file - read on startup, save on SIGxxxx