MINOR: epoll: permit to mask certain specific events

MINOR: epoll: permit to mask certain specific events

A few times in the past we've seen cases where epoll was caught reporting
a wrong event that caused trouble (e.g. spuriously reporting HUP or RDHUP
after a successful connect()). The new tune.epoll.mask-events directive
permits to mask events such as ERR, HUP and RDHUP and convert them to IN
events that are processed by the regular receive path. This should help
better diagnose and troubleshoot issues such as this one, as well as rule
out such a cause when similar issues are reported:

   https://github.com/haproxy/haproxy/issues/2368
   https://www.spinics.net/lists/netdev/msg876470.html

It should be harmless to backport this if necessary.