`eval` has always been used to process the globs in `.yadm/encrypt`.
This is being removed, as there is a risk of executing "dirty" data
found in `.yadm/encrypt`.
Caveats of this change:
* Brace and tilde expansion will no longer work in `.yadm/encrypt`
* Paths with spaces must not be quoted anymore
Overrides are meant to be local to each system, unlike other
configurations which are often added to a user's dotfiles repository.
With this change, the configurations change names:
alt.class => local.class
alt.os => local.os
alt.host => local.host
alt.user => local.user
