reapply exit policy on reload

fix #40676 (closed)

Introduce a new config option ReevaluateExitPolicy to configure the new behavior (defaulting to false).
I followed where ReducedExitPolicy was named to find how to create a new config option and how to document it (but did not fill the changelog).

Tested in chutney by creating a long-lived connection, modifying the exit node torrc, and SIGHUP the exit node, and verify it takes into account the new policy (or not if ReevaluateExitPolicy is unset), and also by setting the policy over the control port.