Small changes around C-stack limit

- Better documentation in 'testes/cstack.lua' about using
'debug.setCstacklimit' to find a good limit.

- Constant LUAI_MAXCSTACK gets added CSTACKERR (extra stack for
error handling), so that it is compatible with the argument to
'debug.setCstacklimit'.

testes/cstack.lua

@@ -4,15 +4,29 @@
 local debug = require "debug"
 
 print"testing C-stack overflow detection"
-
-local origlimit = debug.setCstacklimit(400)
-print("current stack limit: " .. origlimit)
-debug.setCstacklimit(origlimit)
+print"If this test craches, see its file ('cstack.lua')"
 
 -- 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.
+-- overflow. To avoid these errors, you can use the function
+-- 'debug.setCstacklimit' to set a smaller limit for the use of
+-- C stack by Lua. After finding a reliable limit, you might want
+-- to recompile Lua with this limit as the value for
+-- the constant 'LUAI_MAXCCALLS', which defines the default limit.
+-- (The default limit is printed by this test.)
+-- Alternatively, you can ensure a larger stack for the program.
+
+-- For Linux, a limit up to 30_000 seems Ok. Windows cannot go much
+-- higher than 2_000.
+
+
+local origlimit = debug.setCstacklimit(400)
+print("default stack limit: " .. origlimit)
+
+-- change this value for different limits for this test suite
+local currentlimit = origlimit
+debug.setCstacklimit(currentlimit)
+print("current stack limit: " .. currentlimit)
+
 
 local function checkerror (msg, f, ...)
   local s, err = pcall(f, ...)
@@ -104,7 +118,7 @@ do  print("testing changes in C-stack limit")
     return n
   end
 
-  assert(debug.setCstacklimit(400) == origlimit)
+  assert(debug.setCstacklimit(400) == currentlimit)
   local lim400 = check()
   -- a very low limit (given that the several calls to arive here)
   local lowlimit = 38