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.)

ltests.h

@@ -31,7 +31,7 @@
 
 /* compiled with -O0, Lua uses a lot of C stack space... */
 #undef LUAI_MAXCCALLS
-#define LUAI_MAXCCALLS	200
+#define LUAI_MAXCCALLS	400
 
 /* to avoid warnings, and to make sure value is really unused */
 #define UNUSED(x)       (x=0, (void)(x))