In real-world address allocations, disjoint address ranges are common.
Therefore, the host-local allocator should support them.
This change still allows for multiple IPs in a single configuration, but
also allows for a "set of subnets."
Fixes: #45
This change allows the host-local allocator to allocate multiple IPs.
This is intended to enable dual-stack, but is not limited to only two
subnets or separate address families.