Big refactor of the HS client descriptor fetch functionnality. This allows
to fetch an HS descriptor using only a descriptor ID. Furthermore, it's also
possible to provide a list of HSDir(s) now that are used instead of the
automatically choosen one.

The approach taken was to add a descriptor_id field to the rend_data_t
structure so it can be used, if available, by the HS client. The onion
address field however has priority over it that is if both are set, the
onion address is used to fetch the descriptor.

A new public function is introduced called rend_client_fetch_v2_desc(...)
that does NOT lookup the client cache before fetching and can take a list of
HSDirs as a parameter.

The HSFETCH control command now uses this new function thus making it work
and final.

Signed-off-by: David Goulet <dgoulet@ev0ke.net>

As defined in section 4.1.26 in the control-spec.txt, this new event replies
the content of a successfully fetched HS descriptor. This also adds a unit
test for the controller event.

Signed-off-by: David Goulet <dgoulet@ev0ke.net>

This adds the command on the controller side that parses and validate
arguments but does nothing for now. The HS desriptor fetch must be
modularized a bit more before we can use the command.

See control-spec.txt section 3.26 for more information on this command.

Signed-off-by: David Goulet <dgoulet@ev0ke.net>

Signed-off-by: David Goulet <dgoulet@ev0ke.net>

Till someone writes get_interface_address6 interface enumeration that is
routing table aware, these tests will continue to fail on certain
systems because the get_interface_address6() code is broken.

Fixes bug 15591; patch from joelanders

Fixes coverity defect CID 1293337, not in any released version of tor.