nss-myhostname: expose the "outbound" IP addresses under the synthetic "_outbound" hostname

I found myself often looking for a quick way to determine "the local IP
address", and then being lost in the "ip addr" output to find for the
right one to use. This is supposed to help a bit with that. Let's
introduce a new special hostname "_outbound" with semantics similar to
"_gateway" that resolves to addresses that are the closest I could come
up with that maps to "the" local IP address.