Add more local settings

Add some of the more commonly changed settings to a settings.shared file, so that people don't have to worry about their changes to dh-newuser being overwritten.

(Read: I got tired of git overwriting my changes to the dh-newuser file on the devhack machine! Though I did also add the maintainer name and email address to the list of shared settings because they seem like the sort of thing that'd be useful in there. I still need to actually have them be used everywhere other than the dh-newuser script, but at least it's a start.)