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/cstack.lua

@@ -0,0 +1,62 @@
+-- $Id: testes/cstack.lua $
+-- See Copyright Notice in file all.lua
+
+print"testing C-stack overflow detection"
+
+-- Segmentation faults in these tests probably result from a C-stack
+-- overflow. To avoid these errors, recompile Lua with a smaller
+-- value for the constant 'LUAI_MAXCCALLS' or else ensure a larger
+-- stack for the program.
+
+local function checkerror (msg, f, ...)
+  local s, err = pcall(f, ...)
+  assert(not s and string.find(err, msg))
+end
+
+
+do    -- simple recursion
+  local count = 0
+  local function foo ()
+    count = count + 1
+    foo()
+  end
+  checkerror("stack overflow", foo)
+  print("  maximum recursion: " .. count)
+end
+
+
+-- bug since 2.5 (C-stack overflow in recursion inside pattern matching)
+do
+  local function f (size)
+    local s = string.rep("a", size)
+    local p = string.rep(".?", size)
+    return string.match(s, p)
+  end
+  local m = f(80)
+  assert(#m == 80)
+  checkerror("too complex", f, 200000)
+end
+
+
+-- testing stack-overflow in recursive 'gsub'
+do
+  local count = 0
+  local function foo ()
+    count = count + 1
+    string.gsub("a", ".", foo)
+  end
+  checkerror("stack overflow", foo)
+  print("  maximum 'gsub' nest (calls): " .. count)
+
+  -- can be done with metamethods, too
+  count = 0
+  local t = setmetatable({}, {__index = foo})
+  foo = function ()
+    count = count + 1
+    string.gsub("a", ".", t)
+  end
+  checkerror("stack overflow", foo)
+  print("  maximum 'gsub' nest (metamethods): " .. count)
+end
+
+print'OK'