global and subnet reservations

This ticket is about the idea to include in "all" host reservation (HR) mode global reservations:

• not include (current state)
• include (incompatible change)
• include and create a new HR mode for the legacy all value (e.g. "network") which becomes the default (half compatible change)
• introduce a new HR mode (e.g. "every") which includes both global and subnet reservations

There is a comment in the code about the current choice which explains the decision was a bit arbitrary so to reconsider it makes sense. It can also help transition from ISC DHCP where all reservations are syntactically global in the configuration files and the bind between a reservation and a specific subnet is through its optional reserved address.

The default HR mode is important because changing its behavior has a performance impact.

I put the design label as we have to decide what we do and when.