scripts/selinux/mdp: fix initial SID handling

commit e3e0b582 ("selinux: remove unused initial SIDs and improve
handling") broke scripts/selinux/mdp since the unused initial SID names
were removed and the corresponding generation of policy initial SID
definitions by mdp was not updated accordingly.  Fix it.  With latest
upstream checkpolicy it is no longer necessary to include the SID context
definitions for the unused initial SIDs but retain them for compatibility
with older checkpolicy.

Fixes: e3e0b582 ("selinux: remove unused initial SIDs and improve handling")
Signed-off-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>