mx_util: Mention stable sort property in comments

_mx_sort_linked_list and its macro wrapper mx_sort_linked_list currently
do a stable sort. Add comments to the code and the test that this is
guaranteed behaviour, because we are going to depend on it in the
following commits.

mx_util.c

@@ -1268,6 +1268,7 @@ int mx_daemon(int nochdir, int noclose)
     return daemon(nochdir, noclose);
 }
 
+/* guarantee stable sort */
 void _mx_sort_linked_list (void **list,  int (*cmp)(void *o1,void *o2), void ** getnextptr(void *o)) {
 
     void *unsorted=*list;

test_mx_util.c

@@ -489,6 +489,7 @@ static void test_listsort(void)
     }
 
     /* (100 0 1 2 50 50 2 1 0 100)  -> ( 0 0 1 1 2 2 50 50 100 100) stable */
+    /* also verify stable sort */
     for (int i=0;i<10;i++) {
         o[i].next= i==9 ? NULL : &o[i+1];
     }