Fixes for python HBAC bindings

These changes were proposed during a review:
 * Change the signature of str_concat_sequence() to const char *
 * use a getsetter for HbacRule.enabled to allow string true/false and
   integer 1/0 in addition to bool
 * fix a minor memory leak (HbacRequest.rule_name)
 * remove overzealous discard consts

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

Remove dead code from python HBAC bindings

https://fedorahosted.org/sssd/ticket/935

Handle allocation error in python HBAC bindings

https://fedorahosted.org/sssd/ticket/934

HBAC rule validation Python bindings

https://fedorahosted.org/sssd/ticket/943

Since the sysdb is now synchronous and creates its own event context we
don't need an explicit event context anymore in the tools.

This commit completes the migration to a synchronous sysdb

Fixes: #231