TOMOYO: Cleanup part 4.

Gather string constants to one file in order to make the object size smaller.
Use unsigned type where appropriate.
read()/write() returns ssize_t.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: James Morris <jmorris@namei.org>