* Fix skipdata_setup for when _cb is None
ctypes prototype does not accept None value,
so if we want to get a NULL function pointer
then we should either call it with no arguments
or pass zero as an argument.
Fixes#1316
* Do store and return skipdata_setup data
* Add convenience wrappers for skipdata_setup
* Uncomment skipdata_setup tests
* Add alternate usage variants to test_skipdata.py
* document getter
Comparing against 0 or just testing the truthiness is suboptimal, as you
lose the ability to explicitly set the syntax to CS_OPT_SYNTAX_DEFAULT
or 0.
Also, using None to mean "don't change" or other "here is no value"
interpretations is much more pythonic than the C-idiomatic 0.
Nguyen Anh Quynh