Smack: add support for modification of existing rules

Rule modifications are enabled via /smack/change-rule. Format is as follows:
"Subject Object rwaxt rwaxt"

First two strings are subject and object labels up to 255 characters.
Third string contains permissions to enable.
Fourth string contains permissions to disable.

All unmentioned permissions will be left unchanged.
If no rule previously existed, it will be created.

Targeted for git://git.gitorious.org/smack-next/kernel.git

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