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.)

testes/all.lua

@@ -172,6 +172,7 @@ if not _G._soft then
   assert(f() == 'b')
   assert(f() == 'a')
 end
+dofile('cstack.lua')
 dofile('nextvar.lua')
 dofile('pm.lua')
 dofile('utf8.lua')