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.)

luaconf.h

@@ -695,14 +695,14 @@
 /*
 @@ LUAL_BUFFERSIZE is the buffer size used by the lauxlib buffer system.
 ** CHANGE it if it uses too much C-stack space. (For long double,
-** 'string.format("%.99f", -1e4932)' needs 5034 bytes, so a
+** 'string.format("%.99f", -1e4932)' needs 5052 bytes, so a
 ** smaller buffer would force a memory allocation for each call to
 ** 'string.format'.)
 */
 #if LUA_FLOAT_TYPE == LUA_FLOAT_LONGDOUBLE
 #define LUAL_BUFFERSIZE		8192
 #else
-#define LUAL_BUFFERSIZE   ((int)(0x80 * sizeof(void*) * sizeof(lua_Integer)))
+#define LUAL_BUFFERSIZE   ((int)(16 * sizeof(void*) * sizeof(lua_Number)))
 #endif
 
 /*