Initial darwin-multi agent

[avnik]

This effort is a bit incomplete, due lack of mac osx experience and nix-darwin knowledge.

All agents by default run from same user (I have no idea how to generate uids for each agent, as well as have no idea about knowUsers statements and underscored names). But also here a "handle" to assign explicit user to each agent.

Some path is hardcoded (common base directory), as well as usernames.

Both criticizing and advices are welcomed

(This PR replacing #586)

[aciceri]

@roberth We are using this since months without problems