Smack: implement revoking all rules for a subject label

Add /smack/revoke-subject special file. Writing a SMACK label to this file will
set the access to '-' for all access rules with that subject label.

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



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