rename hook: run once per file to be renamed

... as Joey suggested on todo/need_global_renamepage_hook

This hook is applied recursively to returned additional rename
hashes, so that it handles the case where two plugins use the hook:
plugin A would see when plugin B adds a new file to be renamed.

The full set of rename hashes can no longer be changed by hook functions, that
are only allowed to return any additional rename hashes it wants to add.
Rationale: the correct behavior of the recursion would be hard, if not
impossible, to define, if already considered pages were changing on the run.

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