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/basic/basic-forward.h

@@ -97,6 +97,7 @@ typedef enum RuntimeScope RuntimeScope;
 typedef enum TimestampStyle TimestampStyle;
 typedef enum UnitActiveState UnitActiveState;
 typedef enum UnitDependency UnitDependency;
+typedef enum UnitNameMangle UnitNameMangle;
 typedef enum UnitType UnitType;
 typedef enum WaitFlags WaitFlags;