bugfix: don't ask for ->next of an expired circuit

bugfix: don't ask for ->next of an expired circuit
bugfix: keep going when a circ fails in circuit_n_conn_open
        (make circuit_enumerate_by_naddr_nport obsolete)
bugfix: make circuit_n_conn_open only look at circ's that start at us
bugfix: only try circuit_n_conn_open if we're an OP. Otherwise we
        expect connections to always already be up.
bugfix: when choosing path length, pay attention to whether the directory
        says a router is down.
bugfix: when picking good exit, skip routers which are known to be down
        (more work needs to be done on this one)

svn:r838