security/capability.h: cap_issubset/isclear can be boolean

This patch makes cap_issubset/isclear return bool due to these
functions only using either one or zero as their return
value.

No functional change.

Signed-off-by: Yaowei Bai <baiyaowei@cmss.chinamobile.com>
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>