separately. It's important to keep them separate because internal
circuits have their last hops picked like middle hops, rather than like
exit hops. So exiting on them will break the user's expectations.

- Stop cannibalizing internal circuits for general exits, and stop
  cannibalizing exit circuits for rendezvous stuff.

- Don't let new exit streams attach to internal circuits.

- When deciding if we have enough circuits for internal and for exit,
  don't count the wrong ones.

- Treat predicted resolves as predicted port 80 exits.


svn:r5457