Add configuration option for directory authorities to vote on "a" lines
When deploying relays with IPv6 OR port(s) (legacy/trac#4564 (moved)) we will want to make it possible for directory authority operators to select if they should vote on "a" lines or not. Add a configuration option, AuthDirUseIPv6, indicating that a directory authority should test OR ports given in "or-address" lines in server descriptors and vote on them. There might be a better name for the config option.
legacy/trac#5534 (moved) implements the testing and writing "a" lines to status documents. This is done for bridge authorities only. That test should be replaced with a test for the above mentioned config option.