class and subnet pool minor fixes / additions

[tampakrap]

• move the custom class definition on top client classes should be defined before pretty much everything, so that they can be used afterwards by eg a pool definition
• Add allow, deny and max-lease-time options for pool under subnet

[tampakrap]

ping

[0xf10e]

Looks good to me. I don't have any allow/deny statements on the few dhcpds I have though to test this. (Sorry, on the road and can only comment but not "github.review()" from Forkhub)

[aboe76]

@tampakrap thanks for this, @0xf10e no problem