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)