C functions can be tail called, too

A tail call to a C function can have the behavior of a "real" tail
call, reusing the stack frame of the caller.

testes/coroutine.lua

@@ -205,7 +205,7 @@ do
   co = coroutine.create(function () return pcall(foo) end)
   local st1, st2, err = coroutine.resume(co)
   assert(st1 and not st2 and err == 43)
-  assert(X == 43 and Y.name == "pcall")
+  assert(X == 43 and Y.what == "C")
 
   -- recovering from errors in __close metamethods
   local track = {}

testes/errors.lua

@@ -26,7 +26,7 @@ end
 
 local function checkmessage (prog, msg, debug)
   local m = doit(prog)
-  if debug then print(m) end
+  if debug then print(m, msg) end
   assert(string.find(m, msg, 1, true))
 end
 
@@ -289,7 +289,7 @@ end]], "global 'insert'")
 
 checkmessage([[  -- tail call
   return math.sin("a")
-]], "'sin'")
+]], "sin")
 
 checkmessage([[collectgarbage("nooption")]], "invalid option")