Give the Exit flag to Exits that use the secure IRC port 6697

Igor Mitrofano suggests that the Exit flag should be given to relays that allow exiting to the default secure IRC port 6697.

From tor-relays: https://lists.torproject.org/pipermail/tor-relays/2017-July/012585.html

The existing policy is "two ports from [80, 443, 6667]", or these options:

• 80, 443
• 80, 6667
• 443, 6667

We would change to "two ports from [80, 443, 6667-OR-6697]", adding these options:

• 80, 6697
• 443, 6697

We don't need a new consensus method for this, because the way Exit flags are combined stays as majority vote.