T5086: Add sFlow drop-monitor-limit option

hsflowd will export the headers of dropped packets (along with the
name of the function in the Linux kernel where that skb was dropped)
as part of the standard sFlow feed.
This measurement complements the sFlow packet sampling and
counter-telemetry well because it provides visibility into the
traffic that is not flowing.
Very helpful for troubleshooting.
The limit (a rate limit max of N drops per second sent out in the
sFlow datagrams) is the parameter you would set in the CLI.

set system sflow drop-monitor-limit 50