loader: Support halt as well as poweroff to turn off machine

[thj]

Summary

I can't ever remember if loader or ddb wants halt or poweroff - add halt to loader

Test Plan

I've been using this in bhyve for the last week, I don't see why it should behave any differently to the poweroff path. I haven't dug to see if this is the best way to implement this, I'm mostly creating the review as a I tidy a branch

[imp]

I want to rage approve this change.