Rename routerstatus_t.is_running to is_flagged_running

This was the only flag in routerstatus_t that we would previously
change in a routerstatus_t in a consensus. We no longer have reason
to do so -- and probably never did -- as you can now confirm more
easily than you could have done by grepping for is_running before
this patch.

The name change is to emphasize that the routerstatus_t is_running
flag is only there to tell you whether the consensus says it's
running, not whether it *you* think it's running.