Handle expandable values intelligently

* paths: Don't expand %VarName%-style paths.
* addpath: Add the --prepend option to add paths at the beginning of
registry values.
* delpath: Remove all occurrences instead of the first one.
* delenv: Ignore missing registry values.
* setenv: Detect %VarName%-style values and make registry values
expandable automatically.
* addpath: Detect %VarName%-style paths and make registry values
expandable automatically.

Also, I've been messing up left and right since the previous tag.
If you checkout almost any commit since that, it will either not compile
(due to me getting too used to `--ghc-options -w`) or not work properly
(due to me being stupid).
I'll do my best to avoid this shameful experience in the future.