Authentication updates

[kcochran]

In order to improve account security, some changes have been made to how tunnel endpoint updates are authenticated.

Tunnels made after this post now are configured with an "Update Key" (under the "Advanced" tab on the tunnel information page), which is used instead of the general account password when performing automated updates via either the https://ipv4.tunnelbroker.net/ipv4_end.php or the /nic/update (Dyn-alike) mechanisms.  Do not MD5() this value before use.

When an "Update Key" exists, the account password will not work for updates on that tunnel.  Existing tunnels can set an "Update Key" to take advantage of this new mechanism.