core/unit-serialize: fix serialization of markers

Follow-up for ff68472

(cherry picked from commit 5ce8d7d)
(cherry picked from commit 6f30e56)
(cherry picked from commit 4a9f42e)
(cherry picked from commit b030bfb)
(cherry picked from commit bf01f3d)

Author: YHNdnzj

src/core/unit-serialize.c

@@ -39,10 +39,12 @@ static int serialize_markers(FILE *f, unsigned markers) {
         if (markers == 0)
                 return 0;
 
+        bool space = false;
+
         fputs("markers=", f);
         for (UnitMarker m = 0; m < _UNIT_MARKER_MAX; m++)
                 if (FLAGS_SET(markers, 1u << m))
-                        fputs(unit_marker_to_string(m), f);
+                        fputs_with_space(f, unit_marker_to_string(m), /* separator = */ NULL, &space);
         fputc('\n', f);
         return 0;
 }