warn user if RFC4941 IPv6 privacy extensions are enabled

During issue #245 I mentioned that sometimes I get the following error message:
```
ERROR : 99, Cannot assign requested address
[/home/koeppea/dev/ettercap-fixsocket/src/ec_send.c:send_icmp6_echo:685]

 libnet_write: libnet_write_raw_ipv6(): -1 bytes written (Cannot assign requested address)
```

This happened during IPv6 host scan. It took me several days to find out the reason.


The reason for the issue happens only if the following conditions are met:
* ettercap has been compiled with IPv6 support
* a IPv6 router is present triggering SLAAC 
* RFC4941 privacy extensions have been enabled
* the device has returned from hibernation several times

After the return, privacy extensions are still enabled but the interface doesn't have such a address. Only the link-local and the EUI-64 global unicast address.

I believe (have no evidence yet), that there is some issue with the kernel because the error message comes from `write()` trying to write to the socket.

After it happened to me me again and I needed again several hours to remember (just rebooting my device), I decided to do something.

As quite a number of circumstances have to meet, I think the best is just to inform the user about the potential when privacy extensions are enabled.