In order to prepare for the kernel cgroup rework, let's introduce a new
unit type to systemd, the "slice". Slices can be arranged in a tree and
are useful to partition resources freely and hierarchally by the user.
Each service unit can now be assigned to one of these slices, and later
on login users and machines may too.
Slices translate pretty directly to the cgroup hierarchy, and the
various objects can be assigned to any of the slices in the tree.
For "systemctl snapshot" it makes no sense to complete an incomplete
name with ".service" as we previously did, use ".snapshot" instead.
Also, don't bother with mount units or suchlike, we know that this must
be a snapshot and hence is the only sane way for completion.
This makes sure that
systemctl status /home
is implicitly translated to:
systemctl status /home.mount
Similar, /dev/foobar becomes dev-foobar.device.
Also, all characters that cannot be part of a unit name are implicitly
escaped.
Supposed to prevent creating unit files like:
├── dev-sda1.device.wants
│ └── .dot.mount -> /run/systemd/generator/.dot.mount
├── .dot.mount
from:
# cat /etc/fstab
/dev/sda1 /.dot vfat ro 1 3
which we later skip reading because of the leading '.'.