journal: add ability to list values a specified field can take in all entries of the journal
The new 'unique' API allows listing all unique field values that a field specified by a field name can take in all entries of the journal. This allows answering queries such as "What units logged to the journal?", "What hosts have logged into the journal?", "Which boot IDs have logged into the journal?". Ultimately this allows implementation of tools similar to lastlog based on journal data. Note that listing these field values will not work for journal files created with older journald, as the field values are not indexed in older files.
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