nextboot: Reimplement missing -a option

• Reimplement the -a option which was available in the original shell script and is still documented.

• Print the correct usage string when invoked as nextboot.

• Add the -D option to the manual page synopsis.

MFC after: 1 week
Fixes: fd6d47375a78 ("rescue,nextboot: Install nextboot as a link to reboot, rm nextboot.sh")
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D54120