Add a wrapper around config.features that provides the following

Add a wrapper around config.features that provides the following
enhancements:

* The FEATURES variable is automatically synchronized upon modification.

* Modifications result in a permanent override that will cause the change
  to propagate to the incremental stacking mechanism in config.regenerate().
  This eliminates the need to call config.backup_changes() when FEATURES
  is modified, since any overrides are guaranteed to persist despite calls
  to config.reset().

This allows cleanup of all code that overrides FEATURES, and also allows
config.regenerate() to stack FEATURES such that special cases are not
needed for package.env handling inside config.setcpv().