IP allocator to take firstIP and lastIP instead of CIDR block

(Related to https://github.com/docker/docker/issues/15287)

It would be more flexible if the IP allocator could be instructed to use an arbitrary range of addresses, e.g. 10.10.0.100 - 10.10.0.199, rather than be restricted to a block on a CIDR boundary.

In fact, the allocator already works this way internally: newAllocatedMap explodes the passed CIDR range into firstIP and lastIP.

Therefore I think it would be pretty simple for newAllocatedMap to change it to take firstIP and lastIP as arguments instead; with a similar change to RegisterSubnet to become RegisterRange.

RegisterSubnet can then become a wrapper which calls RegisterRange, as a transition measure and/or as an alternate API for callers who have a CIDR block and don't want to calculate firstIP and lastIP themselves.

Start by locating newAllocatedMap and RegisterSubnet in the Go source and trace how the allocator derives firstIP and lastIP from a CIDR range. Confirm the existing allocation behavior, then define the RegisterRange path for explicit bounds while keeping RegisterSubnet as a CIDR-based wrapper. Done means arbitrary ranges work without breaking existing callers.