shell: Display consistent and more useful hostnames.

This is achieved by introducing a new
cockpit.util.hostname_for_display function and using it everywhere.

Also, the transient hostname is no longer used anywhere.  We either
use the static hostname or the name/address that the user has
specified when connecting to a machine.

Fixes #1180
Fixes #1189
daemon: Revert HACK re transient hostname.

We don't need the value of gethostname in the UI anymore so let's just
be a straight proxy for o.f.hostname1.

Also, https://bugs.freedesktop.org/show_bug.cgi?id=83930 is fixed and
we will get change notifications for the Hostname property.
daemon: Don't ever set the transient hostname.

This is not our business.