* If there's an IPv4 and an IPv6 address, return both in the resolved
  cell.
* Treat all resolve requests as permitting IPv6, since by the spec they're
  allowed to, and by the code that won't break anything.

IPv4-only exits have an implicit "reject [::]/0", which was making
policy_is_reject_star() return 1 for them, making us refuse to do
hostname lookups.

This fix chanes policy_is_reject_star() to ask about which family we meant.

We had some old code to send back connected cells for IPv6 addresses,
but it was wrong.  Fortunately, it was also unreachable.

The code previously detected wildcarding and replaced wildcarded
answers with DNS_STATUS_FAILED_PERMANENT.  But that status variable
was no longer used!  Remove the status variable, and instead change
the value of 'result' in evdns_callback.

Thank goodness for compiler warnings. In this case,
unused-but-set-variable.

Thanks to Linus for finding this one.

I have a theory that my tests will work better if the code I'm testing
isn't disabled.

Now, every cached_resolve_t can remember an IPv4 result *and* an IPv6
result.  As a light protection against timing-based distinguishers for
IPv6 users (and against complexity!), every forward request generates
an IPv4 *and* an IPv6 request, assuming that we're an IPv6 exit.  Once
we have answers or errors for both, we act accordingly.

This patch additionally makes some useful refactorings in the dns.c
code, though there is quite a bit more of useful refactoring that could
be done.

Additionally, have a new interface for the argument passed to the
evdns_callback function.  Previously, it was just the original address
we were resolving.  But it turns out that, on error, evdns doesn't
tell you the type of the query, so on a failure we didn't know whether
IPv4 or IPv6 queries were failing.

The new convention is to have the first byte of that argument include
the query type.  I've refactored the code a bit to make that simpler.

Also, fix the function so it actually looks at our ipv6 exit policy.

This uses advertised IPv6 ports as an implicit version check.

This makes it so we can handle getting an IPv6 in the 3 different
formats we specified it for in RESOLVED cells,
END_STREAM_REASON_EXITPOLICY cells, and CONNECTED cells.

We don't cache IPv6 addresses yet, since proposal 205 isn't
implemented.

There's a refactored function for parsing connected cells; it has unit
tests.

This is to avoid confusion with the ipv{4,6}_traffic flags.

These options are for telling the SOCKSPort that it should allow or
not allow connections to IPv4/IPv6 addresses.

These aren't implemented yet; this is just the code to read the
options and get them into the entrey_connection_t.

Also, count ipv6 timeouts vs others.  If we have too many ipv6
requests time out, then we could be degrading performance because of a
broken DNS server that ignores AAAA requests.  Other cases in which
we never learn an AAAA address aren't so bad, since they don't slow
A (ipv4) answers down very much.

Don't advertise an IPv6 exit policy, or accept IPv6 exit requests,
if IPv6Exit is not true.

This is a relatively simple set of changes: we mostly need to
remove a few "but not for IPv6" changes.  We also needed to tweak
the handling of DNS code to generate RESOLVED cells that could get
an IPv6 answer in return.

Now, "accept *:80" means "accept all addresses on port 80", and not
just IPv4.  For just v4, say "accept *4:80"; for just v6 say "accept
*6:80".

We can parse these policies from torrc just fine, and we should be
successfully keeping them out of descriptors for now.

We also now include appropriate IPv6 addresses in "reject private:*"

By default, "*" means "All IPv4 addresses" with
tor_addr_parse_mask_ports, so I won't break anything.  But if the new
EXTENDED_STAR flag is provided, then * means "any address", *4 means
"any IPv4 address" (that is, 0.0.0.0/0), and "*6" means "any IPv6
address" (that is, [::]/0).

This is going to let us have a syntax for specifying exit policies in
torrc that won't drive people mad.

Also, add a bunch of unit tests for tor_addr_parse_mask_ports to test
these new features, and to increase coverage.

Add 'flags' argument to begin cells, per proposal 208.

We'd like these functions to be circuit-relative so that we can
implement a per-circuit DNS cache and per-circuit DNS cache rules for
proposal 205 or its successors.  I'm doing this now, as a part of the
IPv6 exits code, since there are about to be a few more instances
of code using this.