In order to make the OR and dir checking functions in router.c less confusing
we renamed some functions and splitted consider_testing_reachability() into
router_should_check_reachability() and router_do_reachability_checks(). Also we
improved the documentation.
Fixes#18918.
Signed-off-by: Fernando Fernandez Mancera <ffmancera@riseup.net>
