Exits can get the Exit flag without having any ports in their microdescriptor port summary

Almost all clients, relays, and authorities use microdescriptors by default.

Microdescriptor port summaries include a port if it exits to almost all IPv4 addresses (blocks no more than an IPv4 /7).

But the Exit flag is given if at least two of ports 80, 443, 6667 exit to at least an IPv4 /8.

This means an Exit can get the Exit flag, without having any of these ports in its IPv4 exit policy summary.

I suggest we only award the Exit flag if an Exit has at least two of ports 80, 443, 6667 in its IPv4 Exit policy summary.

This also requires a spec change for the Exit flag.