core: add default descriptions for slices

core: add default descriptions for slices

[  OK  ] Created slice system-getty.slice (Slice /system/getty).
[  OK  ] Created slice system-modprobe.slice (Slice /system/modprobe).
[  OK  ] Created slice system-sshd\x2dkeygen.slice (Slice /system/sshd-keygen).
[  OK  ] Created slice user.slice (User and Session Slice).

Before, the first three slices were shown without any description which didn't
look nice.

(cherry picked from commit 4dd21726f852010aef17e9b952b4bb1646fdf496)
(cherry picked from commit 0b0d80d96009e10ce36d683b7991829a2cfca67c)