gnulib-tool: Improve handling of multiple --local-dir options.
* doc/gnulib.texi (Extending Gnulib): Explain how multiple --local-dir
options work.
* gnulib-tool (func_path_prepend): Remove function.
(func_path_foreach): Make IFS handling more robust.
(local_gnulib_path): Collect --local-dir values using func_path_append,
not func_path_prepend.
(func_determine_path_separator): Make IFS handling more robust.
(func_lookup_file_cb): New function.
(func_lookup_file): Rewritten to use func_lookup_file_cb instead of
func_lookup_local_file. Apply the patches in the reverse order of their
origin in $local_gnulib_path.
(func_count_relative_local_gnulib_path): Make IFS handling more robust.
* NEWS: Mention that the first --local-dir option is the one with
highest priority.