CallInfo bit CIST_CLSRET broken in two

Since commit f407b3c4a, it was being used for two distinct (and
incompatible) meanings:
A: Function has TBC variables (now bit CIST_TBC)
B: Interpreter is closing TBC variables (original bit CIST_CLSRET)

B implies A, but A does not imply B.

testes/coroutine.lua

@@ -515,7 +515,7 @@ else
   print "testing yields inside hooks"
 
   local turn
-  
+
   local function fact (t, x)
     assert(turn == t)
     if x == 0 then return 1
@@ -642,7 +642,7 @@ else
 
 
   print "testing coroutine API"
-  
+
   -- reusing a thread
   assert(T.testC([[
     newthread      # create thread
@@ -920,7 +920,7 @@ do   -- a few more tests for comparison operators
     until res ~= 10
     return res
   end
-  
+
   local function test ()
     local a1 = setmetatable({x=1}, mt1)
     local a2 = setmetatable({x=2}, mt2)
@@ -932,7 +932,7 @@ do   -- a few more tests for comparison operators
     assert(2 >= a2)
     return true
   end
-  
+
   run(test)
 
 end
@@ -1037,6 +1037,31 @@ f = T.makeCfunc([[
 	return *
 ]], 23, "huu")
 
+
+do  -- testing bug introduced in commit f407b3c4a
+  local X = false   -- flag "to be closed"
+  local coro = coroutine.wrap(T.testC)
+  -- runs it until 'pcallk' (that yields)
+  -- 4th argument (at index 4): object to be closed
+  local res1, res2 = coro(
+    [[
+      toclose 3   # this could break the next 'pcallk'
+      pushvalue 2   # push function 'yield' to call it
+      pushint 22; pushint 33    # arguments to yield
+      # calls 'yield' (2 args; 2 results; continuation function at index 4)
+      pcallk 2 2 4
+      invalid command (should not arrive here)
+    ]],   -- 1st argument (at index 1): code;
+    coroutine.yield,  -- (at index 2): function to be called
+    func2close(function () X = true end),   -- (index 3): TBC slot
+    "pushint 43; return 3"   -- (index 4): code for continuation function
+  )
+
+  assert(res1 == 22 and res2 == 33 and not X)
+  local res1, res2, res3 = coro(34, "hi")  -- runs continuation function
+  assert(res1 == 34 and res2 == "hi" and res3 == 43 and X)
+end
+
 x = coroutine.wrap(f)
 assert(x() == 102)
 eqtab({x()}, {23, "huu"})
@@ -1094,11 +1119,11 @@ co = coroutine.wrap(function (...) return
           cannot be here!
        ]],
        [[  # 1st continuation
-         yieldk 0 3 
+         yieldk 0 3
          cannot be here!
        ]],
        [[  # 2nd continuation
-         yieldk 0 4 
+         yieldk 0 4
          cannot be here!
        ]],
        [[  # 3th continuation