- Allow to rewrite the request destination behind the client's back.

- Allow to rewrite the request destination behind the client's back.
- Turn the weird-looking unconditional for loop that
  reads the client request into a conditional while loop.
  Move the stuff that only runs once out of the loop.
- Move parts of chat(), server_content_type() and the
  necessary stuff to fix BR#1750917 into get_filter_function().