WIP: new fundamental config CRUD action options

i wasn't planning on working on this yet, but got sidelined into it via a discussion in the mailing list.

there's 8 commits, but pretty tiny overall.

the focus here is on doing some work to help towards the problems of:

1. users running lb config over an existing config, thus modifying it, potentially resulting in stale values from blocked auto-determination.
2. auto/clean files explicitly deleting config files by filename, posing a problem for being able to change the set of files the config is stored in, in future.

i have more ideas in this area that i might explore later, but this implements some small, easy, and significant steps towards solving them.

essentially this boils down to a few new options and using those options in the example auto files.

things are explained in the commit logs, but to give a very brief overview:

• lb config --ignore-saved ignores existing saved options. it can be particularly useful to have in an auto/config file such that whenever you make a change in it, the generated config will not contain stale values, without having to bother first running a command to delete the existing config (like lb clean with an auto file that removes them).
• lb clean --saved-config deletes the saved config files (not the whole config directory). it is particularly useful for auto/clean files to replace directly deleting the files by filename.
• lb config --purge-saved deletes the saved files then exits. the previous command is actually a shortcut to this. this exists also primarily because it does not require elevated privileges. the lb clean redirect is useful also though.

there's also a couple of optimisations that were made as part of this:

• we no longer bother trying to load config and then reprocess args when no config directory exists.
• a more optimal and correct handling of --ignore-system-defaults simply by using a new variable.