selinux: beef up isvalid checks

selinux: beef up isvalid checks

Check that an ID does not refer to a gap in the global array of
definitions.

Constify parameters of isvalid() function and change return type to
bool.

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
Acked-by: Stephen Smalley <stephen.smalley.work@gmail.com>
[PM: merge fixes, dropped boolean checks due to missing dependency]
Signed-off-by: Paul Moore <paul@paul-moore.com>