nextboot: Write nextboot.conf safely

As in the old nextboot.sh script:

• First write everything to a tempfile instead of /boot/nextboot.conf.
• fsync() the tempfile before renaming it to nextboot.conf.

Fixes: fd6d47375a78 ("rescue,nextboot: Install nextboot as a link to reboot, rm nextboot.sh")
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D44572