prop#001: Change server cert verification

  If provided and file doesn't exist, it is a fatal configuration error. If

  provided and the server doesn't accept it, the destination is unusable.

  (optional)

- ``server_cert_fingerprint``: a TLS certificate fingerprint that the server

  must use.  It is a fatal configuration error to specify this but not enable

  ``verify_server_cert``. If not given, the server must use a trusted

  certificate. (optional)

- ``verify_server_cert``: whether to verify the server certificate or not.

  Default yes. If enabled and ``server_cert_fingerprint`` is not given, it must

  be trusted (as determined by the local machine's configuration outside of

  sbws). If enabled and ``server_cert_fingerprint`` is given, the certificate

  from the server must have the specified fingerprint. If enabled but neither

  of those things are true, the server is unusable. (optional)

- ``verify_server_cert``: either a boolean or a path to a file. If yes (the

  default), the server's certificate must be trusted (as determined by the

  local machine's configuration outside of sbws). If no, do no verification of

  the certificate at all. If a path to a file and the file does not exist, it

  is a fatal configuration error. Otherwise, the certificate the server users

  must be present in the file pointed to by this option. (optional)

Example: CDN

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Example: Private Local Destination

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Here, an authority has decided he doesn't want to trust anyone but themself.

They are running 2 relays on the same machine as a webserver that only they

will use.

HTTPS is not technically required to protect credentials flowing over the

Internet. In fact, the webserver isn't even reachable from the Internet!

This authority chooses to use a client TLS certificate to identify their

scanner(s), so their webserver must use HTTPS.

However, the authority wants to make sure only their sbws scanner(s) can

connect to this webserver, so they technically set up HTTPS. On their webserver

they generate a self-signed certificate. On the sbws scanner side, they *could*

choose to specify the fingerprint of this TLS certificate with

``server_cert_fingerprint``, but instead trust themself to keep their

infrastructure secure and forego verification of the server certificate

entirely.

On their webserver they generate a self-signed certificate.

On the sbws scanner side, they *could* choose to assume everything will be okay

and his server will not change certificates. But they're paranoid, so they get

a copy of the server's certificate and store it in a local file.

.. todo:: What file format?

::

    relays = AAAA...AAAA, BBBB...BBBB

    relay_section_method = uniform_random

    url = https://33.33.33.33:4433/sbws.bin

    client_cert = ${paths:sbws_home}/secure_bwauth_client.cert

    verify_server_cert = off

    client_cert = ${paths:sbws_home}/secure_bwauth_scanner.cert

    verify_server_cert = ${paths:sbws_home/secure_bwauth_server.cert

Example: "Borrow" bandwidth from unsuspecting mirrors

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