formalize valid requirements specifications

From the discussion [here](https://github.com/jonboulle/fleet/commit/1b42558dad5e378954893669e57e25d1d804115d#commitcomment-6982201)

Basically, from the set of all possible fleet requirements:
- `X-ConditionMachineID` 
- `X-ConditionMachineBootID` (can we scrap this?)
- `X-ConditionMachineOf`
- `X-Conflicts`
- `X-ConditionMachineMetadata`
- `X-MachineMetadata` (can we scrap this?)
- `X-MemoryReservation` (gone)
- `X-DiskReservation` (gone)
- `X-CoresReservation` (gone)

... we need to define which subsets are valid, and I strongly favour blocking invalid permutations at the earliest point (e.g. `fleetctl submit`, API submit)

For example, `X-ConditionMachineID` cannot be used with `X-*Reservation`, or `X-FleetConflicts`, or `X-ConditionMachineOf`. Effectively, `X-ConditionMachineID` triggers a "do not perform any scheduling for this job" condition, and hence is suitable for use by upstream schedulers.

This is an area where we're consistently failing to follow go style, which prefers shorter variable names