This time, I follow grarpamp's suggestion and move the check for
.exit+AllowDotExit 0 to the top of connection_ap_rewrite_and_attach,
before any rewriting occurs.  This way, .exit addresses are
forbidden as they arrive from a socks connection or a DNSPort
request, and not otherwise.

It _is_ a little more complicated than that, though.  We need to
treat any .exit addresses whose source is TrackHostExits as meaning
that we can retry without that exit.  We also need to treat any
.exit address that comes from an AutomapHostsOnResolve operation as
user-provided (and thus forbidden if AllowDotExits==0), so that
transitioning from AllowDotExits==1 to AllowDotExits==0 will
actually turn off automapped .exit addresses.