bb8a3296e8
In principle, arbitrary notifications may be sent via sd_notify. But in practice, this is not useful at all, since the manager only accepts notifications from services and ignores anything except a few specific ones. The others will be logged if debugging is enabled. OTOH, the manager produces EXIT_STATUS, but nothing in systemd looks at it, which is rather confusing. So remove the recommendation to use X_ prefixes, and instead say that other messages will be ignored. Also, mention that mkosi uses this. Having an example may be useful to understand what is going on. Strangely, this is the first reference to mkosi in our man pages. Even more strangely, debian is the only place which hosts the mkosi man page (among the sites we have definitions for), so I linked to that version.
System and Service Manager
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