allow use of default certificate store

[Ludwig Nussel]

While trying to fix up one of our tools to support proper https I
found the following features missing from pyopenssl:

- it's not possible to pass a directory as certificate store. On
  openSUSE we have all trusted ca certificates in individual files
  in /etc/ssl/certs
- applications should not need to know about /etc/ssl/certs so the
  lib should just use SSL_CTX_set_default_verify_paths() to take the
  location built into the distro's openssl.

The attached patch implements both features without changing the
API.

[Jean-Paul Calderone (exarkun)]

I've added support for a second optional argument to load_verify_locations and added a separate set_default_verify_paths method based on your patch. I think together these changes expose all the functionality you're interested in. I took this approach because I prefer to mimic the OpenSSL API as closely as possible. The code is still in the branch in the "Related branches" section, not trunk. Please let me know what you think, and hopefully I can merge this to trunk soon.

[Ludwig Nussel (l-n)]

fine with me, thanks! :)

[Jean-Paul Calderone (exarkun)]

Alright, took a while, but I finally merged the branch.