Move system-upgrade.target.requires mounts into a shell script

redhat-upgrade-tool handles media (--iso, --device) mounts using systemd
requires, which isn't going to work since the system-upgrade isn't
started via systemd. Write a shell script to perform the mount, instead,
and run scripts in /var/lib/system-upgrade/upgrade-prep early in
upgrade-init.

Resolves: rhbz#1094193
New version 0.7.13