scanner: introduce "public-code" and "private-code"

The options are used to indicate how the code will be used - will it be
public, as part of a DSO or private.

In nearly every instance, people want to use the latter. One noticeable
exception is the wayland libraries. They provide the base marshalling
protocol that everyone uses.

The option "code" was deprecated in favour of "public-code" with a
warning message produced to guide people.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>