ifconfig: functions for tunnel creation and manipulation

Create, change and delete sit, ipip and gre tunnels.
sit: dbus factory service

Also contains service functions for changing and deleting sit tunnel.
ipip: dbus factory service

Also contains service functions for changing and deleting ipip tunnels.
gre: dbus factory service

Also contains service functions for changing and deleting gre tunnels.
client: add support for parsing general tunnel attributes

Needed for sit, ipip and gre tunnels.
client: parse tunnel ifcfg files and generate xml configs