MINOR: lua: Get the action return code on the stack when an action finishes

MINOR: lua: Get the action return code on the stack when an action finishes

When an action successfully finishes, the action return code (ACT_RET_*) is now
retrieve on the stack, ff the first element is an integer. In addition, in
hlua_txn_done(), the value ACT_RET_DONE is pushed on the stack before
exiting. Thus, when a script uses this function, the corresponding action still
finishes with the good code. Thanks to this change, the flag HLUA_STOP is now
useless. So it has been removed.

It is a mandatory step to allow a lua action to return any action return code.