Tor should lower-case its BridgeDistribution string

See comment:8 in legacy/trac#32547 (moved), where phw tells a bridge operator that they need to set BridgeDistribution to "none", because using "None" is not right.

That isn't a good user experience, and it's easy to fix.

One option is that inside check_bridge_distribution_setting() when we do if (!strcmp(bd, RECOGNIZED[i])) we should do strcasecmp instead. That is, don't complain if it's a recognized value and the only issue is that it's not all lowercase.

But a better option imo is that we lowercase it in place first, so that if the user types None, bridgedb still sees none.