classifier: Prepare for "struct cls_rule" needing to be destroyed.

classifier: Prepare for "struct cls_rule" needing to be destroyed.

Until now, "struct cls_rule" didn't own any data outside its own memory
block.  An upcoming commit will make "struct cls_rule" sometimes own blocks
of memory, so it needs "destroy" and to a lesser extent "clone" functions.
This commit adds these in advance, even though they are mostly no-ops, to
make it possible to separately review the memory management.

Signed-off-by: Ben Pfaff <blp@nicira.com>