gnulib-tool: Change --import. New options --add/remove-import.

gnulib-tool: Change --import. New options --add/remove-import.

* gnulib-tool: New options --add-import, --remove-import.
(func_usage): Document them.
(have_associative): Define always.
(func_import): In import mode, don't merge the specified settings with
the cached settings. Implement remove-import mode.
* doc/gnulib-tool.texi (Modified imports): Mention the new options.
Explain when to use them versus --import.
(Simple update): Use --add-import instead of --import.
* NEWS: Mention the change.