Security update

[iblue]

This should improve the security of nsupdate-client in two ways:

• Fetch IP address not via http (which is susceptible to MITM attacks)
• Validate the fetched IP address, before including it in the update to prevent sending arbitrary updates in case the server at bot.whatismyipaddress.com git compromized.

There is still an exploitable race condition by using /tmp/$REC.tmp which can be exploited by creating this file first, but the attacker needs to be authenticated on the machine running nsupdate-client, so I can live with that for now.