revamp btrfs subvolumes
revamp btrfs subvolumes

- in keeping with systemd @var/lib/machines btrfs subvolume:
  - s,var-lib-ex,var/lib/ex
  - s,var-tmp,var/tmp
- add nodatacow mount option to
  - @home
  - @srv
  - @var/lib/ex
  - @var/lib/postgres
  - @var/log
    - add nodatacow option from `sub mount-btrfs-subvolume` instead of
      from `method !disable-cow`
  - @var/spool
  - @var/tmp
- add nodev,noexec,nosuid mount option to
  - @var/log
  - @var/spool

atweiden/holovault