rescue,nextboot: Install nextboot as a link to reboot, rm nextboot.sh

[imp]

Summary

Reboot now emulates the nextboot shell script completely. Retire the
nextboot.sh script and install the link. Retain the same manual page,
since there's enough differences between nextboot and reboot that
talking about nextboot would likely be confusing in nextboot.8

Sponsored by: Netflix

[kib]

Do you need to do something with /rescue?

[imp]

In D43831#1000047, @kib wrote:

Do you need to do something with /rescue?

Yes I do. It's easy to fix. I'll add a review for it..

Also, rescue + nextboot is currently broken for zfs anyway. The nextboot script used zfsbootcfg, but that was never added to rescue when the call was added to nextboot.sh. Yet another loose end I'm cleaning up.

[imp]

Note: Folding this with D43843 on push.