Clean up and fix exit policy check in connection_exit_connect().

Previously, we would reject even rendezvous connections to IPv6
addresses when IPv6Exit was false.  But that doesn't make sense; we
don't count that as "exit"ing.  I've corrected the logic and tried
to make it a lottle more clear.

Fixes bug 18357; this code has been wrong since 9016d9e8 in
0.2.4.7-alpha.