Bug fixes and afc2 access in cython bindings

This commit fixes several compile errors I encountered and adds the ability
to use "com.apple.afc2" for jailbroken devices using the cython bindings.
In afc.pxi:
- Add Afc2Client subclass for "com.apple.afc2" access.
- Add read() method to AfcFile
- Initialize some pointers to NULL at time of declaration

In lockdown.pxi:
- Make LockdownServiceDescriptor inherit from Base, since it calls
  self.handle_error
- Explicitly cast ssl_enabled when calling lockdownd_start_session