Fix up sound driver proc-reading interfaces.

Fix up sound driver proc-reading interfaces.

The old ones were not only hard to use, they were in fact
impossible or a low-level sound driver to get right, since
the core sound code didn't even export all the necessary
information.

Add the file offset to the /proc read interfaces, and make
them simpler to use. The core now does a offset updates, and
sanity-checks the values so that the low-level drivers don't
need to worry.