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 4dd21726)
(cherry picked from commit 0b0d80d9)
(cherry picked from commit 3fa82957)