Support for multiple DHCP servers

[torbbang]

dhcpProxy should have support for relaying DHCP packets to multiple upstream DHCP servers. This is neccesary as having multiple DHCP servers is common practice.

I have a half-way functioning prototype of this and will be finishing it in the next few days.

[bentole]

Sorry about my butter fingers closing this issue. My initial thought was if HA is needed you would have multiple instances of this proxy running at different IPs and then having multiple ip helper statements (Cisco), but there might be other use cases and pros I'm not seeing? :)