core: Add hostname manager (bgo #699458)

Provides a mechanism to get and set the transient system hostname
and persistent static hostname, as well as be notified when these
change.

Uses the DBus service org.freedesktop.hostname1, with fallback to
get/sethostname and /etc/hostname.

https://bugzilla.gnome.org/show_bug.cgi?id=699458
keyfile: Add hostname deprecation warning