address: parse sockaddr based link addresses
logging: add logging guard macro

Used later on for tunnel discovery tracing.
iftype: use proper ipip tunnel name for interface type

s/NI_IFTYPE_TUNNEL/NI_IFTYPE_IPIP/g for easier identification. Please
note that ARPHRD_TUNNEL remains as is.
iflist: tunnel discovery and trace

Skeleton code for sit, ipip and gre tunnel discovery. Also a debug
function to dump various IFLA_IPTUN/IFLA_GRE data.
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.
Merge pull request #234 from kmroz/tunnel-dev3