Introduce an internal priority for chains

Introduce an internal priority for chains

For better handling of the sorting of chains introduce an internally used
priority. Use a lookup table to store the priorities. For now their actual
values do not matter just that the values cause the chains to be properly
sorted through changes in the following patches. However, the values are
chosen as negative so that once they are sorted along with filtering rules
(whose priority may only be positive for now) they will always be instantiated
before them (lower values cause instantiation before higher values). This
is done to maintain backwards compatibility.

Signed-off-by: Stefan Berger <stefanb@linux.vnet.ibm.com>