core: add default descriptions for slices

[  OK  ] Created slice system-getty.slice (Slice /system/getty).
[  OK  ] Created slice system-modprobe.slice (Slice /system/modprobe).
[  OK  ] Created slice system-sshd\x2dkeygen.slice (Slice /system/sshd-keygen).
[  OK  ] Created slice user.slice (User and Session Slice).

Before, the first three slices were shown without any description which didn't
look nice.

src/core/slice.c

@@ -180,6 +180,14 @@ static int slice_load(Unit *u) {
         if (r < 0)
                 return r;
 
+        if (!u->description) {
+                _cleanup_free_ char *tmp = NULL;
+
+                r = unit_name_to_path(u->id, &tmp);
+                if (r >= 0)  /* Failure is ignored… */
+                        u->description = strjoin("Slice ", tmp);
+        }
+
         return slice_verify(s);
 }