users: Drop dependency on chfn. (#260)

Pass the full name directly to useradd instead, using the "-c" (comment)
parameter, which is "currently used as the field for the user's full
name" according to the documentation.

The chfn utility is no longer installed by default on current Fedora
releases due to its dependency on libuser. (They split out chfn and chsh
from util-linux into an optional subpackage.) It could be added as a
dependency of Calamares, but since it is needed inside the chroot, it
has to be on the base live image, not the overlay. Thus, to allow
testing Calamares with a simple "dnf install calamares" again, the
dependency needs to go away. And it is unnecessary anyway.