Moss State: `moss state activate` setting the default entry

[sammypanda]

Expected

When attemtping to rescue a broken system, it's ¿likely/reasonable? to want to:

1. go into the rescue environment (in this case a bootable drive)
2. mount and copy the path of ur root drive, for using the moss -D option
3. listing your states to see which might be in a working state based on the date shown and the date you last had a working system
4. try to, from here, activate it and then reboot expecting it to automatically go into the one you "activated"

this is especially relevant if your last working state doesn't is after the cut off in the boot list.

Recommended

Having moss state activate set the default entry.

relevant to #312