networking: Hostname viewing & setting

addon/network settings/hostname

allowing the hostname for the system to be set by the user using
the preferences gui

Change-Id: I676c368070ef89a20435f97ee39e3caa25abf8f9
Reviewed-on: https://review.haiku-os.org/c/870
Reviewed-by: Axel Dörfler <axeld@pinc-software.de>
Reviewed-by: Stephan Aßmus <superstippi@gmx.de>