tunneling: tunnel data struct and helper functions

Basic structures and functions for sit, tunnel (ipip) and gre tunnels.
netdev: tunneling functions

sit, tunnel (ipip) and gre interface structures added to netdev.
get and set function provided to handle interfaces mentioned above.
iflist: tunnel discovery

Discovery for sit, ipip and gre tunnels.
dbus: sit tunnel dbus service

No factory yet. Purely read-only service.
dbus: ipip dbus service

No factory yet. Purely read-only service.
dbus: gre tunnel dbus service

No factory yet. Purely read-only service.
iflist: check IFLA_LINKINFO before parsing

Not doing so can lead to a segfault.