Reintroduce ExitType

This introduces `ExitType=main|cgroup` for services.
Similar to how `Type` specifies the launch of a service, `ExitType` is
concerned with how systemd determines that a service exited.

- If set to `main` (the current behavior), the service manager will consider
  the unit stopped when the main process exits.

- The `cgroup` exit type is meant for applications whose forking model is not
  known ahead of time and which might not have a specific main process.
  The service will stay running as long as at least one process in the cgroup
  is running. This is intended for transient or automatically generated
  services, such as graphical applications inside of a desktop environment.

Motivation for this is #16805. The original PR (#18782) was reverted (#20073)
after realizing that the exit status of "the last process in the cgroup" can't
reliably be known (#19385)

This version instead uses the main process exit status if there is one and just
listens to the cgroup empty event otherwise.

The advantages of a service with `ExitType=cgroup` over scopes are:
- Integrated logging / stdout redirection
- Avoids the race / synchronisation issue between launch and scope creation
- More extensive use of drop-ins and thus distro-level configuration:
  by moving from scopes to services we can have drop ins that will affect
  properties that can only be set during service creation,
  like `OOMPolicy` and security-related properties
- It makes systemd-xdg-autostart-generator usable by fixing [1], as obviously
  only services can be used in the generator, not scopes.

[1] https://bugs.kde.org/show_bug.cgi?id=433299

docs/TRANSIENT-SETTINGS.md

@@ -307,6 +307,7 @@ Most service unit settings are available for transient units.
 ✓ ExecStartPre=
 ✓ ExecStop=
 ✓ ExecStopPost=
+✓ ExitType=
 ✓ FileDescriptorStoreMax=
 ✓ GuessMainPID=
 ✓ NonBlocking=