rename hook: instead of modifying the passed-by-name array, return a copy

rename hook: instead of modifying the passed-by-name array, return a copy

This is intended to solve Joey's concerns expressed on
http://ikiwiki.info/todo/need_global_renamepage_hook/, i.e. the need to make it
possible to use this hook from external plugins.

A plugin using this hook still can add/modify/remove elements of the
@torename array.

Signed-off-by: intrigeri <intrigeri@boum.org>