Ability to override default open files limit

[aleksystasiak]

Haproxy does calculate processes open file soft limit using maxconn value. This value is far too low in large cache scenario.
It is possible to change hard limit through setting LimitNoFile in systemd service file, the problem is haproxy refuses to continue working after reaching soft limit and ignores hard limit. This leads to setting maxonn to ridiculously high values.
It would be great to have ability to override this default behaviour. (or change the way haproxy sets it which is ~ maxconn * 2)