Small changes around C-stack limit

- Better documentation in 'testes/cstack.lua' about using
'debug.setCstacklimit' to find a good limit.

- Constant LUAI_MAXCSTACK gets added CSTACKERR (extra stack for
error handling), so that it is compatible with the argument to
'debug.setCstacklimit'.

lstate.c

@@ -388,7 +388,7 @@ LUA_API lua_State *lua_newstate (lua_Alloc f, void *ud) {
   preinit_thread(L, g);
   g->allgc = obj2gco(L);  /* by now, only object is the main thread */
   L->next = NULL;
-  g->Cstacklimit = L->nCcalls = LUAI_MAXCSTACK;
+  g->Cstacklimit = L->nCcalls = LUAI_MAXCSTACK + CSTACKERR;
   g->frealloc = f;
   g->ud = ud;
   g->warnf = NULL;