datatype: maintain table of all datatypes and add registration/lookup function

datatype: maintain table of all datatypes and add registration/lookup function

Add a table containing all available datatypes and registration/lookup functions.
This will be used to associate a stand-alone set in the kernel with the correct
type without parsing the entire ruleset.

Additionally it would now be possible to remove the global declarations for the
core types. Not done yet though.

Signed-off-by: Patrick McHardy <kaber@trash.net>