NTP Management Modification

[heaton84]

This is a nice library, almost exactly what I was looking for!

I made a couple of changes to suit my needs, namely:

1. I added method to read variables from an NTP server via control packets. My angle is to store a timezone variable in the NTP server, and have all my clocks get the timezone data from that server. Example usage:

Timezone localTime;

bool updateTimezone()
{
  String r;
  if (readNTPVariable("TZ", r))
  {
    localTime.setPosix(r.c_str());
    
    return true;
  }
  return false;
}

2. I also modified queryNTP to respect the server poll if our current poll is set too quickly. This is a courtesy to NTP server owners.