Change offset(nan) and sync(0) to WARNING

[rodrigogansobarbieri]

Due to network instability in some environments,
the NTP algorithm may discard all servers/peers
temporarily, triggering CRITICAL Nagios alert
"CRITICAL: offset is out of range (nan)".

There are two problems with that alert:

1. The message is misleading, as offset=nan does
   not mean anything. The worst metric selection
   hides the real alert "CRITICAL: No sync peer selected".
   Therefore, the worst metric selection list is reversed,
   as when "No sync peer selected" happens,
   "offset is out of range (nan)" always happens as well.

2. The problem is usually transient and does not
   require immediate attention. It may go away with
   network workload changes, or may persist until the
   root cause of the issue is investigated. Given that it
   is of "CRITICAL" level, it obfuscates other much more
   critical alerts from NTPmon and also Nagios. Therefore,
   the WARNING level is more appropriate.

Closes #5

[paulgear]

@rodrigogansobarbieri If it's OK with you, I'm going to close this PR now. I've fixed the NaN problem in a different way, and the downgrading of sync failure from CRITICAL to WARNING is not a change which I'm willing to consider now, since the whole point of NTP is to sync the clock. I think the reversal of the list in worst_metric() is worth thinking about a bit further, and I've created issue #11 to track this.