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.)

llimits.h

@@ -184,11 +184,13 @@ typedef LUAI_UACINT l_uacInt;
 ** must be an unsigned with (at least) 4 bytes (see details in lopcodes.h)
 */
 #if LUAI_BITSINT >= 32
-typedef unsigned int Instruction;
+typedef unsigned int l_uint32;
 #else
-typedef unsigned long Instruction;
+typedef unsigned long l_uint32;
 #endif
 
+typedef l_uint32 Instruction;
+
 
 
 /*