systemd: Use name in unit status messages

Currently, unit status messages are logged like this in the journal:

Started Journal Service.
Finished Load Kernel Modules.
Starting Anonymizing overlay network for TCP...

With this change, the messages are logged like this instead:

Started systemd-journald.service.
Finished systemd-modules-load.service.
Starting tor@default.service...

This makes it easier to search for unit status messages in the journal by pasting the unit name.

Closes #19568 (closed)