IsSystemdService: make check stricter to avoid false positives

[tmds]

Checking the INVOCATION_ID envvar is causing false positives.
Instead, we now return true only when the direct parent process is 'systemd'.

Fixes #2525

cc @Tragetaschen @anurse @onyxmaster @Tratcher

[analogrelay]

Technically breaking so we should run this by the API/Breaking Change review group (cc @aspnet/asp-net-api-reviews). I think it's warranted though (and the set of people it would break is pretty small: scenarios where INVOCATION_ID is set, the direct parent is not systemd, and the desired behavior is to act as a systemd service).

[analogrelay]

LGTM, should be mergable once we get approval for the breaking change.

[halter73]

LGTM too

[halter73]

Thanks @tmds! @anurse do you think we should backport this?

[analogrelay]

The fact that it is technically a breaking change makes me a little nervous. But it is customer-reported and has no workaround. We also strongly believe there is no intended scenario that would be broken.

Let's take it to Tactics. @halter73 could you open a backport PR? I can fill in the justification details.