bootc-fstab-edit: 2 small improvements

[champtar]

• fixup_etc_fstab: comment / when using defaults

If we 'upgrade' to an image without composefs or if we runtime
disable composefs, remounting ro will break the system.
If option field is `defaults` lets comment the line instead.

• bootc-fstab-edit: improve service dependencies

Looking at the target definitions:
- multi-user.target Requires basic.target
- basic.target Requires sysinit.target
- sysinit.target Wants local-fs.target
- local-fs.target is After local-fs-pre.target

local-fs-pre.target is wanted by some services, but it's indirect,
so it's cleaner to be WantedBy local-fs.target and Wants local-fs-pre.target.

While at it add Conflicts=shutdown.target.