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)