Simplify comitup/network connection feedback logic

[pguyot]

Comitup callback script could be replaced with something sending a packet for nabd to process.

printf '{"type":"comitup","callback":"%s"}', "$1"

Likewise, network connectivity test on transition to idle should be:

• replaced with a mix of network tests, as TCP on port 53 can be blocked (e.g. ICMP, hostname resolution, HTTP, HTTPS)
• happen much less frequently and on a regular basis
• be paired with a pipe on a command such as "ip monitor link" to monitor link changes (that would trigger a new connectivity test).

Eventually, both systems should send events to services that need to connect to the internet, such as nabweatherd and nabairqualityd.