Add the hs_config.{c|h} files contains everything that the HS subsystem needs
to load and configure services. Ultimately, it should also contain client
functions such as client authorization.

This comes with a big refactoring of rend_config_services() which has now
changed to only configure a single service and it is stripped down of the
common directives which are now part of the generic handler.

This is ground work for prop224 of course but only touches version 2 services
and add XXX note for version 3.

Signed-off-by: David Goulet <dgoulet@torproject.org>