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

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

The nextboot.sh script no longer exists, so doesn't need to be fixed up
to create rescue. However, now we need a link from nextboot to reboot.

Sponsored by: Netflix
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D43831
Differential Revision: https://reviews.freebsd.org/D43843