soft-reboot: Many changes

soft-reboot: Many changes

- Add --reboot and --reset arguments
- Don't compile on centos stream 9 (missing `open_tree` glibc wrapper)
  as the functionality isn't supported by systemd there; that said
  we should also do dynamic detection
- Fix /sysroot writability
- If we target as soft reboot a deployment
  *other* than the staged one, automatically clear the staged
  deployment as otherwise the semantics are too confusing.
- Rename the APIs so they all say `soft_reboot` and not `next_root`

Signed-off-by: Colin Walters <walters@verbum.org>