Create concept of "global units"

[jonboulle]

This feature has been requested several times in one form or another. The general use case is "run one copy of a particular unit on every single machine in the cluster".

In implementation terms, the basic idea is that there is a static manifest of unit in fleet's registry (etcd), and every fleet agent watches this central depository, cloning and starting units locally. Hence, it completely bypasses the fleet scheduling engine.

Various supporting changes will be necessary before this can be implemented.

[jonboulle]

Also relevant: #494

[jonboulle]

Fixed by #786. Needs documenting for proper use as part of forthcoming docpocalypse.