On distro openSUSE Tumbleweed and SUSE Linux Enterprise the path
of command usermod is not in environment variable PATH, we need to
use absolute path to find command.
Username policies differ across the distributions. See the discussion on:
https://gitlab.gnome.org/GNOME/gnome-control-center/merge_requests/359
It is not possible to hard-code some rules here without the need for
downstream modifications. Let's check the validity of usernames dynamically
over "usermod" to prevent this.
Just be careful that this is based on undocumented usermod behavior, which
may change in the future.
The `round_image()` function has the `icon_size` parameter, which is not
used by that function, because the size is determined from a pixbuf size.
Let's remove that redundant `icon_size` parameter.
