libsmack: reorganize rule lists for struct smack_accesses

Until now rule list was implemented as single linked list of all rules
added into struct smack_accesses. This patch breaks this list into several
lists, with one list of rules per subject label.
Label dictionary (added in cd2ce11) is now merged with smack_accesses (it's
abstraction layer is dropped). Each element in array of labels describe
single label and all rules when it's a subject are added into this label's
list.

This data structure is close to internal kernel data structures for Smack
rules. This allows for slightly better performance because rules are
grouped by subject.
More importantly though, this patch prepares ground for rule merging.

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