Improve debug logging of rlm_perl

Don't just show the attributes being copied, but also say where they are
copied from/to. This results in output like this:

```
Debug: (11032)  perl : <-- RAD_REPLY
Debug: (11032)  perl : <--   Tunnel-Private-Group-Id = 123
Debug: (11032)  perl : <--   EAP-Message = 0x030a0004
Debug: (11032)  perl : <-- RAD_CHECK
Debug: (11032)  perl : <--   Auth-Type = EAP
Debug: (11032)  perl : <-- RAD_REQUEST
Debug: (11032)  perl : <--   User-Name = anonymous
Debug: (11032)  perl : <--   NAS-IP-Address = 127.0.0.1
```