When we refactored purpose_needs_anonymity(), we made it so _all_
bridge requests required anonymity.  But that missed the case
that we are allowed to ask a bridge for its own descriptor.

With this patch, we consider the resource, and allow "authority.z"
("your own descriptor, compressed") for a bridge's server descriptor
to be non-anonymous.

Fix for bug 20410; bug not in any released Tor.

Closes #20435

Signed-off-by: David Goulet <dgoulet@torproject.org>

I believe that this should never trigger, but if it does, it
suggests that there was a gap between is_sensitive_dir_purpose and
purpose_needs_anonymity that we need to fill.  Related to 20077.

Conflict in entrynodes.c: any_bridge_supports_microdescriptors was
removed in master, and modified in 19858_v2

Patch from pingl; patch for 13827.

"ours" commit to avoid bumping version.

("ours" merge to avoid taking version bump)

This helps protect against bugs where any part of a buf_t's memory
is passed to a function that expects a NUL-terminated input.

It also closes TROVE-2016-10-001 (aka bug 20384).