podman: add darwin support with machine management

[delafthi]

• restructure module from podman-linux to platform-agnostic podman
• move linux-specific implementation to modules/services/podman/linux/
• add darwin module with declarative machine management
• implement launchd-based watchdog for auto-starting machines
• maintains backward compatibility with existing linux functionality

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[n-hass]

I've just had a skim and will take a more thorough look later. Thanks for the contribution.

I don't believe it's conventional to namespace service settings under the platform the feature is available for, eg the services.podman.darwin.machines. Instead give the user a useful error if they configure a machine on a non-Darwin system.

Any thoughts anyone else?

[khaneliman]

I agree I don't really like darwin namespacing really... Should be able to hide options / disable options on darwin if they can't be used.

          visible = pkgs.stdenv.hostPlatform.isLinux;
          readOnly = pkgs.stdenv.hostPlatform.isDarwin;

or just don't make them available through not importing ?

[delafthi]

Any thoughts anyone else?

I agree. An implementation without a darwin namespace would be nicer.