journal: implement message catalog
The message catalog can be used to attach short help texts to log lines, keyed by their MESSAGE_ID= fields. This is useful to help the administrator understand the context and cause of a message, find possible solutions and find further related documentation. Since this is keyed off MESSAGE_ID= this will only work for native journal messages. The message catalog supports i18n, and is useful to augment english language system messages with explanations in the local language. This commit only includes short explanatory messages for a few example message IDs, we'll add more complete documentation for the relevant systemd messages later on.
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