Ignore sysvinit scripts when systemd is used

[remicalixte]

What does this PR do?

use SYSTEMCTL_SKIP_SYSV env var

Motivation

Remove irrelevant warning on install

[arbll]

Let's add a small comment explaining why we need SYSTEMCTL_SKIP_SYSV=true

[remicalixte]

Yes, sure

[olivielpeau]

what's the warning? Some customers use the enable command directly, could the warning mislead them?

[remicalixte]

Something like this:

insserv: warning: current start runlevel(s) (empty) of script `datadog-agent' overrides LSB defaults (2 3 4 5).
insserv: warning: current stop runlevel(s) (0 1 2 3 4 5 6) of script `datadog-agent' overrides LSB defaults (0 1 6).

But they also appear when enabling are disabling core system services like dbus.service or networking.service so it shouldn't confuse customers too much

[olivielpeau]

@remicalixte ok, thanks for the info. I could still see these warnings confuse some users (they look a bit worrying TBH).

Could you update the release note on the sysv-init support to mention that these warnings can be safely ignored on systemd-based systems? (and could you check our public docs as well?). Thanks!

[remicalixte]

@olivielpeau the public docs on debian advice to use the service command which is available with sysvinit: https://docs.datadoghq.com/agent/basic_agent_usage/deb/?tab=agentv6.
Is there any other public docs I would have missed ?

[olivielpeau]

@remicalixte you can double-check on the docs available on this repo as well, but that's all I can think of 👍

[remicalixte]

Missed some docs mentioning sysvinit on this repo