userboot: bump the interface to v6, add an accept_interpreter callback

[kevans]

Summary

If an explicit loader wasn't requested, then bhyveload(8) maintains a
/boot handle that it can use for swapping to a different flavor. This
means that we expose all of the host /boot to the sandbox for the
duration of script execution.

Add a callback to ack that we're OK with the interpreter so that
bhyveload(8) can release the bootfd. This is worth doing because it's
prior to guest script execution, so we're still running a reasonably
untainted process.