Add AF_NETLINK to set of allowed socket address families

Add AF_NETLINK to set of allowed socket address families

contrib/unbound{,_portable}.service.in:
With the changes introduced in f6a527c25ad2e60e2dc129fff3605e6ec48c30f2
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