Rename GVFS protocol endpoint

There are two issues to consider here before renaming the endpoint:

• We'll need to make sure there is a period of time that the client understands both endpoints, so that services have time to execute the rename
• If services ever drop support for the old endpoint, that will render older clients unusable.

Given the second issue, and given that over time we expect to whittle down the gvfs endpoint as more functionality shows up in the native git protocol, we need to evaluate if this rename is worth doing.

A less disruptive approach that we could consider is to lock down the current protocol but preserve it, and only add any new functionality to a new, renamed endpoint.

No files, tests, or implementation entry points are named. Start by evaluating the compatibility and migration concerns described in the issue, then determine whether renaming the endpoint is worthwhile or whether the existing endpoint should be preserved while new functionality uses a renamed endpoint; done means reaching that decision.