Patch to cleanup getbuffer() and convertbuffer():
 - getbuffer() doesn't call convertbuffer() if pb->bf_getbuffer==NULL. If pb->bf_getbuffer==NULL, PyObject_GetBuffer() fails and so the call to convertbuffer() is useless.
 - convertbuffer() calls getbuffer() to check that the buffer is 'C" contiguous (and to factorize the code)
 - release the buffer if the buffer is not contigous => fix a bug
 - rename "errmsg" and "buf" to "expected" to reuse converterror() term
 - Remove /* XXX Really? */: I don't understand the comment and the code looks ok

The main change is that convertbuffer() now requires a "C" contiguous buffer. That change concerns "s#", "y", "z" and "t#" formats.

If a function would like to support non contiguous buffers, it should use "O" format and then PyObject_GetBuffer(). I don't think that builtin Python functions do support non contiguous buffers.