backend.list -j now outputs IPs/port information

[gquintard]

since DNS resolution happens only once for static backends and can be a bit obscure for dynamic backend, it's valuable to be able to dump the IPs that are used.

In order to not break existing scripts which may varnishadm backend.list, only update the JSON ouput

side note: it's a bit confusing that vbe_list needs to produce a JSON value ending with a coma if pflag is true, but not if it's false. Do we want to change that? And also maybe make printing the probe_details and probe_message a responsibilty of the backend implementation, rather than splitting the key-value pair across two functions?