BUG/MEDIUM: lua: the function txn_done() from action wrapper can crash

BUG/MEDIUM: lua: the function txn_done() from action wrapper can crash

If an action wrapper stops the processing of the transaction
with a txn_done() function, the return code of the action is
"continue". So the continue can implies the processing of other
like adding headers. However, the HTTP content is flushed and
a segfault occurs.

This patchs add a flag indicating that the Lua code want to
stop the processing, ths flags is forwarded to the haproxy core,
and other actions are ignored.

Must be backported in 1.6