main: note that we want to set int 31h segment to DOS DS

This commit does not change the behaviour of the code
as it already did the right thing. However, it notes
that that should happen. (This is DR DOS compatible.
For example, SHSUCDX depends on DR DOS setting this
segment so that it can keep track of the DOS DS
across it being relocated. We do not yet relocate it
but keeping a reference here is still useful.)

kernel/main.c

@@ -270,7 +270,7 @@ STATIC void setup_int_vectors(void)
 
   /* install default handlers */
   for (i = 0x23; i <= 0x3f; i++)
-    setvec(i, empty_handler);
+    setvec(i, empty_handler); /* note: int 31h segment should be DOS DS */
   HaltCpuWhileIdle = 0;
   for (pvec = vectors; pvec < vectors + (sizeof vectors/sizeof *pvec); pvec++)
     setvec(pvec->intno, (intvec)MK_FP(FP_SEG(empty_handler), pvec->handleroff));