Eliminate code duplication between installer and athena-renumber

[geofft]

Both the installer and athena-renumber in debathena-recovery-mode-config have code to resolve an IP address. The installer's is more featureful, taking a name too, and also does not suffer from e.g. #1069. We should really use the same code in both places.

Unfortunately I can't think of _how_ to do this cleanly, short of making a single source package that spits out debathena-recovery-mode-config and debathena-installer-udeb as binary packages.