Fix retval vs. retval confusion: retval is meant to store the (userland)
pointer where data is to be returned by ibask() (currently unimplemented),
while retval holds the value returned by the libgpib ibfoo() functions.

The confusion resulted in the ibfoo() functions returning an uninitialized
value except in situations where the GPIB activity has been terminated
abnormally.

MFC after: 3 days