The configuration entry store that is used for configuration files needs
to keep track of all entries in two different structures:

- a singly linked list is being used to be able to iterate through
  configuration files in the order they have been found

- a string map is being used to efficiently look up configuration
  entries by their key

This store is thus something that may be used by other, future backends
as well to abstract away implementation details and iteration over the
entries.

Pull out the necessary functions from "config_file.c" and moves them
into their own "config_entries.c" module. For now, this is simply moving
over code without any renames and/or refactorings to help reviewing.