libsmack: reorganize rule lists for struct smack_accesses

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.
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>