BUG/MINOR: listener/threads: always use atomic ops to clear the FD events

There was a leftover of the single-threaded era when removing the
FD_POLL_HUP flag from the listeners. By not using an atomic operation
to clear the flag, another thread acting on the same listener might
have lost some events, though this would have resulted in that thread
to reprocess them immediately on the next loop pass.

This should be backported as far as 1.8.

src/listener.c

@@ -799,7 +799,7 @@ void listener_accept(int fd)
 					 * a while in case it comes back. In the mean time, we need to
 					 * clear this sticky flag.
 					 */
-					fdtab[fd].ev &= ~FD_POLL_HUP;
+					_HA_ATOMIC_AND(&fdtab[fd].ev, ~FD_POLL_HUP);
 					goto transient_error;
 				}
 				fd_cant_recv(fd);