Fix python HBAC bindings for python <= 2.4
    
    Several parts of the HBAC python bindings did not work with old Python
    versions, such as the one shipped in RHEL5.
    
    The changes include:
    * a compatibility wrapper around python set object
    * PyModule_AddIntMacro compat macro
    * Py_ssize_t compat definition
    * Do not use PyUnicode_FromFormat
    * several function prototypes and structures used to have "char
      arguments where they have "const char *" in recent versions.
      This caused compilation warnings this patch mitigates by using
      the discard_const hack on python 2.4