Rewrite update-po target, so that it works less like a shell script and more

Rewrite update-po target, so that it works less like a shell script and more
like a makefile with real dependencies.

Instead of overwriting the old po file, write the new one to .po.new.  This is
less annoying and integrates better with the NLS web site.

Also, we can now merge languages that don't have a po file yet, by merging
against all other po files of that language, to pick up recurring translations
automatically.  This previously only worked when a po file already existed.