The Scan function's interface to the split function was not sufficient
to handle an empty final token in a pure function; state was required.
This was ugly.

We introduce a special error value that a split function can return
that signals that this token is OK, but is the last one and scanning
should stop immediately _after_ this token.

The same effect could be achieved using the same trick (a special
error value) and checking for that error after Scan finishes, but it's
a little clumsy. Providing a published sentinel value in bufio is
cleaner and means everyone can use the same trick. The result
is an error-free scan.

Rewrite the test (that was only barely working) to use the value
and be more robust.

Also write a new example showing how to do it.

Fixes #11836

Change-Id: Iaae77d0f95b4a2efa0175ced94d93c66353079e8
Reviewed-on: https://go-review.googlesource.com/14924Reviewed-by: Ian Lance Taylor <iant@golang.org>