hostname: Fallback to transient hostname

hostname: Fallback to transient hostname

After pretty hostname, and static hostname, also support transient
hostname as a last resort before 'BlueZ X.XX'.

This happens on Fedora's Workstation installation as it calls
"hostnamectl set-hostname" on startup. In Fedora Silverblue, the default
hostname is set as fedora in /etc/os-release.

In both cases, we should fall back to that transient hostname, as bad as
it could be.

Note that the transient hostname needs to be monitored through the
kernel directly, as explained in:
https://www.freedesktop.org/software/systemd/man/org.freedesktop.hostname1.html