service: fix main processes exit behavior for type notify services

service: fix main processes exit behavior for type notify services

Before this commit, when the main process of a Type=notify service exits the
service would enter a running state without passing through the startup post
state. This meant ExecStartPost= from being executed and allowed follow-up
units to start too early (before the ready notification).
Additionally, when RemainAfterExit=yes is used on a Type=notify service, the
exit status of the main process would be disregarded.

After this commit, an unsuccessful exit of the main process of a Type=notify
service puts the unit in a failed state. A successful exit is inconsequential
in case RemainAfterExit=yes. Otherwise, when no ready notification has been
received, the unit is put in a failed state because it has never been active.
When all processes in the cgroup of a Type=notify service are gone and no ready
notification has been received yet, the unit is also put in a failed state.