[Shtoom] local ip determination

[Zooko O'Whielacronx]

Should sip.Call.setLocalIP() be refactored to use the new 
nat.getLocalIPAddress()?

By the way, you might be able to glean useful ideas or code from this 
venerable and battle-tested module:

http://mnetproject.org/repos/mnet/egtp/ipaddresslib.py

I don't really know most of what is in that hairy module, but I do know 
that it worked well enough on all supported platforms at Mojo Nation 
and then Mnet for many years.

The authors Greg Smith and Hauke Johannknecht (with help from others) 
have my respect for their network fu.

--Z