journalctl-vl-server: allow querying units for specific uid in system scope

This also moves journal_add_unit_matches() back to
journalctl-filter.[ch], as I see more appropriate.

src/shared/shared-forward.h

@@ -26,6 +26,7 @@ typedef enum DnssecMode DnssecMode;
 typedef enum Fido2EnrollFlags Fido2EnrollFlags;
 typedef enum KeySourceType KeySourceType;
 typedef enum LabelFixFlags LabelFixFlags;
+typedef enum MatchUnitFlag MatchUnitFlag;
 typedef enum MountInNamespaceFlags MountInNamespaceFlags;
 typedef enum NamePolicy NamePolicy;
 typedef enum OutputFlags OutputFlags;