Tunnel dev - readonly dbus services for sit, ipip and gre tunnels

DO NOT MERGE - DO NOT MERGE - DO NOT MERGE

Current TODO's (discussion with mt):

s/NI_IFTYPE_TUNNEL/NI_IFTYPE_IPIP/ + s/"tunnel"/"ipip" in naming functions
instead of void * netdev_get|set_tunnel... create separate get/set for sit/ipip/gre
guard tunnel tracing calls per IRC conversation
!tb[IFLA_LINKINFO] != error -> keep the guard, but output a debug instead and return 0