hostnamed,shared/hostname-setup: expose the origin of the current hostname

hostnamed,shared/hostname-setup: expose the origin of the current hostname

In hostnamed this is exposed as a dbus property, and in the logs in both
places.

This is of interest to network management software and such: if the fallback
hostname is used, it's not as useful as the real configured thing. Right now
various programs try to guess the source of hostname by looking at the string.
E.g. "localhost" is assumed to be not the real hostname, but "fedora" is. Any
such attempts are bound to fail, because we cannot distinguish "fedora" (a
fallback value set by a distro), from "fedora" (received from reverse dns),
from "fedora" read from /etc/hostname.

/run/systemd/fallback-hostname is written with the fallback hostname when
either pid1 or hostnamed sets the kernel hostname to the fallback value. Why
remember the fallback value and not the transient hostname in /run/hostname
instead?
We have three hostname types: "static", "transient", fallback".
– Distinguishing "static" is easy: the hostname that is set matches what
  is in /etc/hostname.
– Distingiushing "transient" and "fallback" is not easy. And the
  "transient" hostname may be set outside of pid1+hostnamed. In particular,
  it may be set by container manager, some non-systemd tool in the initramfs,
  or even by a direct call. All those mechanisms count as "transient". Trying
  to get those cases to write /run/hostname is futile. It is much easier to
  isolate the "fallback" case which is mostly under our control.
And since the file is only used as a flag to mark the hostname as fallback,
it can be hidden inside of our /run/systemd directory.

For https://bugzilla.redhat.com/show_bug.cgi?id=1892235.