release: Use unprivileged mode for disc1 and memstick

This same change was made for the bootonly and mini-memstick targets in
commit cbba69fb5793. That uncovered some makefs bugs, which are now
fixed. As there are no other known issues, extend this to disc1 and
memstick images now.

The change for the dvd image is waiting on PR 283387.

Reviewed by: brooks, cperciva
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50179