BUG/MINOR: hlua: Properly enable/disable line receives from HTTP applet

BUG/MINOR: hlua: Properly enable/disable line receives from HTTP applet

From a lua HTTP applet, in the getline() function, we must take care to
disable receives when a line is retrieved and to restart receives when the
function is called again. In addition, when an applet execution is finished,
we must restart receives to properly drain the request.

This patch could help to fix #3293. It must be backported to 3.3. On older
version, no bug was reported so we can wait a report first. But in that
case, hlua_applet_http_recv() should also be fixed (on 3.3 it was fixed
during the applets refactoring).