Fixes to zsys_dir_create and zsys_dir_delete

* The create function was not accurately making multi-directory path
* zfile was depending on incorrect behavior
* Added support for printf formatting in zsys_dir_create/delete
* Added better selftest for zsys_dir_create/delete
Improved certificate serialization API

* Often we want to save/load with a directory path and a filename
  constructed from pieces. This is easier done by passing them as
  a printf format than asking the caller to create the full path
  name.
* Modified zcert_load/save/save_public
* Modified zcertstore_new in same way
Finished security examples
Merge pull request #232 from hintjens/master

Finishing up with security documentation, and examples