manager: add 'needs-stop/needs-start' markers

Useful for packaging scripts, when units are removed.
When multiple markers are assigned without +/-, the last one wins.
When using +/-, the job merging logic is followed to the extent possible.

src/basic/unit-def.h

@@ -62,6 +62,8 @@ typedef enum FreezerState {
 typedef enum UnitMarker {
         UNIT_MARKER_NEEDS_RELOAD,
         UNIT_MARKER_NEEDS_RESTART,
+        UNIT_MARKER_NEEDS_STOP,
+        UNIT_MARKER_NEEDS_START,
         _UNIT_MARKER_MAX,
         _UNIT_MARKER_INVALID = -EINVAL,
 } UnitMarker;