Authorities on IPv6: minor fixes and unit tests

Update the code for IPv6 authorities and fallbacks for function
argument changes.

Update unit tests affected by the function argument changes in
the patch.

Add unit tests for authority and fallback:
 * adding via a function
 * line parsing
 * adding default authorities
(Adding default fallbacks is unit tested in #15775.)

changes/feature17327

@@ -0,0 +1,5 @@
+  o Minor feature (IPv6):
+    - Add a flag ipv6=address:orport to the DirAuthority and FallbackDir torrc
+      options. Add hard-coded ipv6 addresses for directory authorities with
+      ipv6 lines in their descriptors.
+      Closes ticket 17327; patch from Nick Mathewson / "teor".