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/coroutine.lua

@@ -107,7 +107,7 @@ function filter (p, g)
   end)
 end
 
-local x = gen(100)
+local x = gen(80)
 local a = {}
 while 1 do
   local n = x()
@@ -116,7 +116,7 @@ while 1 do
   x = filter(n, x)
 end
 
-assert(#a == 25 and a[#a] == 97)
+assert(#a == 22 and a[#a] == 79)
 x, a = nil