machine/resource: add basic resource reservation information

This exposes basic resource reservation information in MachineState. The
current approach for calculating free resources is extremely simple:
assume a fixed block of resources (HostResources) is reserved for usage
outside of fleet units (e.g. by system processes and fleet itself), and
subtract the aggregate resources that have been reserved explicitly
using fleet directives.
Merge pull request #622 from jonboulle/resources

Basic resource reservations