Configuration File Protection
  Portage includes a system for configuration file protection which means ebuilds
  don't have to worry about accidentally clobbering files in /etc. This is
  known as 'protection', and it is controlled by the CONFIG_PROTECT and
  CONFIG_PROTECT_MASK variables.
  Any directory which is listed in CONFIG_PROTECT (and any subdirectories
  thereof), except for any which are listed in CONFIG_PROTECT_MASK (and
  subdirectories) are automatically 'protected' by Portage when copying an image
  from DESTDIR to ROOT. Rather than installing protected files directly,
  Portage will install them as ._cfg0000_filename. These can then be processed
  by the etc-update or dispatch-conf files at the user's discretion.
  Packages must not attempt to override this system via pkg_postinst or
  similar. If you need a file renamed, removed or changed in a particular way, you
  should display a message informing the user.