MacPorts needs a general config file update mechanism

[blb@…]

When a MacPorts install is upgraded, the config files are not overwritten to avoid stomping on local changes. This has the disadvantage that new features which can be configured via *.conf aren't added to the current configuration.

The best method to handle this would be with a general method to update configuration files by simply adding the new feature-specific items to the currently-existing files. Otherwise we end up with scripts for specific purposes (e.g., upgrade_sources_conf_default.tcl) which in the long term will become messy and difficult to maintain.

[afb@…]

See also vintage #2365

[blb@…]

In case I was a bit vague, I'm specifically referring to MacPorts' own config files (macports.conf, sources.conf, etc).