basic: reorder UnitType enum

The enum order will be used to order jobs in the job queue.
Make sure that unit types that fork aditional processes come first to
maximize parallelism.

src/basic/unit-def.h

@@ -5,14 +5,17 @@
 
 #include "macro.h"
 
+/* The enum order is used to order unit jobs in the job queue
+ * when other criteria (cpu weight, nice level) are identical.
+ * In this case service units have the hightest priority. */
 typedef enum UnitType {
         UNIT_SERVICE = 0,
+        UNIT_MOUNT,
+        UNIT_SWAP,
         UNIT_SOCKET,
         UNIT_TARGET,
         UNIT_DEVICE,
-        UNIT_MOUNT,
         UNIT_AUTOMOUNT,
-        UNIT_SWAP,
         UNIT_TIMER,
         UNIT_PATH,
         UNIT_SLICE,