MINOR: lua: Create the global 'act' object to register all action return codes

MINOR: lua: Create the global 'act' object to register all action return codes

ACT_RET_* code are now available from lua scripts. The gloabl object "act" is
used to register these codes as constant. Now, lua actions can return any of
following codes :

  * act.CONTINUE for ACT_RET_CONT
  * act.STOP for ACT_RET_STOP
  * act.YIELD for ACT_RET_YIELD
  * act.ERROR for ACT_RET_ERR
  * act.DONE for ACT_RET_DONE
  * act.DENY for ACT_RET_DENY
  * act.ABORT for ACT_RET_ABRT
  * act.INVALID for ACT_RET_INV

For instance, following script denied all requests :

  core.register_action("deny", { "http-req" }, function (txn)
      return act.DENY
  end)

Thus "http-request lua.deny" do exactly the same than "http-request deny".