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

Marius,

Per yesterdays' mail:

I've resolved nanny screaming about setting ttl, tos and pmtudisc in sit/gre/ipip.

Still seeing: ni_error("%s: Unable to discover interface details", ...) in __ni_discover_tunneling() when creating a SIT interface via `ip tunnel add ...`. As I mentioned in the mail, discovery ultimately gives correct results for SIT interfaces, just on creation we are dropped into our discovery code with an tb[IFLA_LINKINFO] == NULL. I will verify what happens when creation is done through wicked to see if it's a `ip tunnel ..` problem.

Feel free to reject/place a hold.