Refactor resolve_my_address() so logs are more accurate / helpful

It returns the method by which we decided our public IP address
(explicitly configured, resolved from explicit hostname, guessed from
interfaces, learned by gethostname).

Now we can provide more helpful log messages when a relay guesses its IP
address incorrectly (e.g. due to unexpected lines in /etc/hosts). Resolves
ticket 2267.

While we're at it, stop sending a stray "(null)" in some cases for the
server status "EXTERNAL_ADDRESS" controller event. Resolves bug 8200.