Changes in the control of C-stack overflow

* unification of the 'nny' and 'nCcalls' counters;
  * external C functions ('lua_CFunction') count more "slots" in
    the C stack (to allow for their possible use of buffers)
  * added a new test script specific for C-stack overflows. (Most
    of those tests were already present, but concentrating them
    in a single script easies the task of checking whether
    'LUAI_MAXCCALLS' is adequate in a system.)

lparser.c

@@ -367,10 +367,12 @@ static void adjust_assign (LexState *ls, int nvars, int nexps, expdesc *e) {
 }
 
 
-#define enterlevel(ls)	luaE_incCcalls((ls)->L)
+/*
+** Macros to limit the maximum recursion depth while parsing
+*/
+#define enterlevel(ls)	luaE_enterCcall((ls)->L)
 
-
-#define leavelevel(ls)	((ls)->L->nCcalls--)
+#define leavelevel(ls)	luaE_exitCcall((ls)->L)
 
 
 /*