Added macro 'luaL_pushfail'

The macro 'luaL_pushfail' documents all places in the standard libraries
that return nil to signal some kind of failure. It is defined as
'lua_pushnil'. The manual also got a notation (@fail) to document those
returns. The tests were changed to be agnostic regarding whether 'fail'
is 'nil' or 'false'.

testes/api.lua

@@ -698,7 +698,7 @@ for k, v in ipairs(t) do
   assert(v1 == v and p)
 end
 
-assert(debug.getuservalue(4) == nil)
+assert(not debug.getuservalue(4))
 
 debug.setuservalue(b, function () return 10 end, 10)
 collectgarbage()   -- function should not be collected