spec: fix certain symlink targets
Symlinks under legacy directories which point outside the set of legacy
directories break after the usrmerge operation and have to be fixed,
both at runtime and in the package.
The systemd package has 2 symlinks which need attention to work on
merged-usr hierarchies:
'/usr/lib/environment.d/99-environment.conf' -> '../../../etc/environment'
'/usr/sbin/mount.ddi' -> '../../usr/bin/systemd-dissect'
Set the correct targets and add a conflict on unmerged-usr filesystem.