Use Agent API for locking mirrors and known_hosts

[DrJosh9000]

If the Agent API experiment is enabled and working, then shell will now use it for locking instead of flock.

At least two reasons for this:

1. The API socket can be bind-mounted into a container and "just work", whereas flock is harder to make work correctly across a container boundary.
2. Locking and unlocking in the Agent API has no disk cost, so can be polled more frequently, so should be slightly faster.

[DrJosh9000]

Perhaps the choice of "lock providers" should be configurable?

Oooh. What about making it a hook?