/rescue is a separate set now

[maya%NetBSD.org@localhost]

Hi folks,

I've split out /rescue into its own set.
This is intended to make updates safer. It's not necessary to update it
at all, and if something goes wrong in your update, you can boot
/rescue/init and recover, as described here:
https://www.netbsd.org/docs/current/#recovering-via-rescue

I had to change a lot of installers and images in distrib/, I hope I
didn't miss one. Let me know if one doesn't install /rescue now.