Add an explicit ipv4/ipv6 switch for ImmortalCoord<->App local communication

[rrnewton]

Right now there are various places where it's assumed that Windows=>ipv6 and Linux=>ipv4 for the communication between the app process and ImmortalCoordinator process.

These should not be conflated. Instead, there should be an explicit flag somewhere for how the coordinator and app communicate. Indeed, perhaps it should be over multiple interfaces (not just localhost) or offer other configurability.

Any configurability for ImmortalCoordinator faces the question of whether it comes as a command-line parameter to the coordinator (late), or as part of the Ambrosia RegisterInstance upload to the metadata table in the cloud (early).

It's a discussion that deserves it's own issue -- shouldn't these ultimately be fungible? Config parameters could be put in either place. Adding an ipv4/v6 switch could go in one place or the other for the time being.