Instead, count exits as usable if they have the exit flag, and
present if they also have a non-reject exit policy.

Requiring a threshold of usable descriptors avoids directories trickling
exit descriptors to clients to discover their ExitNodes settings.

Part of 27236.

Part of 26979.

Closes 26979.

To avoid future duplicates.

Part of 26979.

Part of 26979.

Part of 26979.

That's what git does for tor.

Part of 26979.

Part of 26979.

Implements 27275.

test.c no longer uses lround(), so we don't need to declare it,
and we can use math.h for fabs().

Fixes bug 27185; bugfix on 0.2.2.2-alpha.

Previously, Tor would only check the exit flag. In small networks, Tor
could bootstrap once it received a consensus with exits, without fetching
the new descriptors for those exits.

After bootstrap, Tor delays descriptor fetches, leading to failures in
fast networks like chutney.

Fixes 27236; bugfix on 0.2.6.3-alpha.

Unlike the old test, this test no will no longer mess around with
the forbidden internals of any openssl data structures.

Additionally, it verifies several other behaviors of
tor_tls_cert_matches_key() that we had wanted to verify, such as
the possibility of the certificate's key not matching.

Fixes bug 27226; bugfix on 0.2.5.1-alpha.

This allows us to mock our own tor_tls_get_peer_certificate()
function in order to test ..cert_matches_key(), which will in turn
allow us to simplify test_tortls_cert_matches_key() considerably.

Prep work for the fix for 27226.