add_default_ip extras

[aerusso]

Hello! I've been using these two patches locally, and thought they might be useful for others.

The first adds the ability to force the use of a link-local address. This can be useful when publishing services that can only be used on a LAN, or alternatively for some "split-horizon" dns setups.

The second one provides a mechanism to prepend ssh and some key-file options before the calls made to ip. This allows for several useful setups that can be otherwise challenging, such as using ddupdate to manage the dns entries for a system that cannot run ddupdate, centralizing all dns updates on a single machine, or using a router's ip command to infer a global ipv4 address instead of calling out to a remote server with addr_default_web.py.