"tune.maxpollevents" global parameter was not limited. It was possible to
set any integer value. But this value is used to allocate the array of
events used by epoll. With a huge value, it seems the allocation silently
fail, making haproxy totally unresponsive.
So let's to limit its value to 1 million. It is pretty high and it should
not be an issue to forbid greater values. The documentation was updated
accordingly.
This patch could be backported to all stable branches.