Hostname management is complicated. At least, how it is implemented currently.
For example, NMPolicy also sets the hostname (NMPolicy calls
nm_settings_set_transient_hostname() to have hostnamed set the hostname,
but then falls back to sethostname() in settings_set_hostname_cb()).
Also, NMManager tracks the hostname in NM_MANAGER_HOSTNAME too, and
NMPolicy listens to changes from there -- instead of changes from
NMSettings.

Eventually, NMHostnameManager should contain the hostname parts from NMSettings
and NMPolicy.