Refactor make-memstick.sh to avoid creating the 'dangerously
dedicated' partition scheme, reported to cause the memstick.img
to fail to boot.

Similar to how make-memstick.sh worked on stable/8, use makefs(8)
to create the actual filesystem. Then calculate the size of the
resulting image file, create the GPT partition scheme, then dd(1)
the filesystem created with makefs(8) to the freebsd-ufs GPT
partition.

This was tested on a known-working machine[1] for regression, and
a known-not-working machine[2] to ensure the boot issue has been
resolved.

Testers: myself [1], db [2]
MFC After: 1 week
Sponsored by: The FreeBSD Foundation