Fix __retval vs. retval confusion: retval is meant to store the (userland)

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.

Implement more of __ibsta: END and SRQI status bits (taken out of the
uPD7210 IRQ status).

Approved by:	re (kib)

git-svn-id: svn://svn.freebsd.org/base/stable/8@216573 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f