rcutil: add a method to convert environment variables to config items Handling config and environ priorities has been messy. Partially because we don't have config layers - you either get all configs (sys + user), or none. Ideally, environ like $EDITOR, $PAGER should be able to override the system configs "ui.editor", "pager.pager". This patch provides the ability to convert them into config items, so they can be inserted into the middle config layer between system rc and user rc.