When parsing addreses via Tor's internal DNS lookup API:
* reject IPv4 addresses in square brackets (with or without a port),
* accept IPv6 addresses in square brackets (with or without a port), and
* accept IPv6 addresses without square brackets, as long as they have no port.
This change completes the work started in 23082, making address parsing
consistent between tor's internal DNS lookup and address parsing APIs.
Fixes bug 30721; bugfix on 0.2.1.5-alpha.
