Allow to specify the "snmp engine" start stime

[vigno88]

It could be cumbersome to track the boot counts of the snmp notifier on all platform, thus the boot count of the snmp engine is stuck at zero. When the os restart the uptime restart from zero and the boot count doesn't increment, which doesn't respect the RFC (https://www.ietf.org/rfc/rfc2574.txt):

2.2. Replay Protection

Each SNMP engine maintains three objects:

• snmpEngineID, which (at least within an administrative domain)
  uniquely and unambiguously identifies an SNMP engine.

• snmpEngineBoots, which is a count of the number of times the
  SNMP engine has re-booted/re-initialized since snmpEngineID
  was last configured; and,

• snmpEngineTime, which is the number of seconds since the
  snmpEngineBoots counter was last incremented.

I thus suggest to add a new parameter such as "snmp-engine-start-time", that would be used a a reference time to compute the "snmpEngineTime" (instead of the default uptime)

[maxwo]

That is interesting and worth investigating to find a good solution.

I will see if there is a way to have a reliable date or boot count to use, but until then, specify it from the command line can be an option, true.

[vigno88]

Good, I will open a PR in the coming days with this feature.

Thanks