API refactoring: introduce probe context

Introduce a "probe context" to allow passing the "ip" context from the
probe function to the get_value callback used by the bytecode
interpreter. This enables the "ip" context to be used from the filter
bytecode and from the capture bytecode.

Also fix signedness of the return type of the get_value context
callbacks for cases where the type is unsigned.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I2cb40a9b3bb34c42392bb663368e5af360554dbb