Define unique_ptr specialization for Py_buffer.

Define unique_ptr specialization for Py_buffer.

This patch causes PyBuffer_Release() to be called when the associated
buffer goes out of scope.  I've been using it as follows:

 ...
 Py_buffer_up buffer_up;
 Py_buffer py_buf;

 if (PyObject_CheckBuffer (obj)
     && PyObject_GetBuffer (obj, &py_buf, PyBUF_SIMPLE) == 0)
   {
      /* Got a buffer, py_buf, out of obj.  Cause it to released
         when it goes out of scope.  */
     buffer_up.reset (&py_buf);
   }
   ...

This snippet of code was taken directly from an upcoming patch to
python-value.c.

gdb/ChangeLog:

	* python/python-internal.h (Py_buffer_deleter): New struct.
	(Py_buffer_up): New typedef.