hostname: split out hostname management from NMSettings
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.
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