libsmack: merge rules with the same subject and object before applying them

libsmack: merge rules with the same subject and object before applying them

All rules with the same subject and object will be merged into a single one
before applying rules to kernel or writing them to a file. The result will
consist of smaller number of rules, but they will have the same semantics.
This enhances performance greatly when there are a lot of rules to merge.
The merging code has negligible overhead when there are no merges to
perform.

Signed-off-by: Rafal Krypa <r.krypa@samsung.com>