SELinux: export object class and permission definitions

Moves the definition of the 3 structs containing object class and
permission definitions from avc.c to avc_ss.h so that the security
server can access them for validation on policy load. This also adds
a new struct type, defined_classes_perms_t, suitable for allowing the
security server to access these data structures from the avc.

Signed-off-by: Chad Sellers <csellers@tresys.com>
Acked-by:  Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: James Morris <jmorris@namei.org>