contrib/unbound{,_portable}.service.in:
With the changes introduced in f6a527c25a
it is now necessary to also allow access to the AF_NETLINK socket
address family to be able to get information from interfaces.
Without the AF_NETLINK address family the systemd service errors with:
```
error: failed to list interfaces: getifaddrs: Address family not
supported by protocol
```
Fixes#350
