BUG/MINOR: config: Limit "tune.maxpollevents" parameter to 1000000

BUG/MINOR: config: Limit "tune.maxpollevents" parameter to 1000000

"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.