BACKPORT: selinux: use separate table for initial SID lookup

This moves handling of initial SIDs into a separate table. Note that the
SIDs stored in the main table are now shifted by SECINITSID_NUM and
converted to/from the actual SIDs transparently by helper functions.

This change doesn't make much sense on its own, but it simplifies
further sidtab overhaul in a succeeding patch.

Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
Reviewed-by: Stephen Smalley <sds@tycho.nsa.gov>
[PM: fixed some checkpatch warnings on line length, whitespace]
Signed-off-by: Paul Moore <paul@paul-moore.com>

(cherry picked from commit 24ed7fdae669feda4c5e0dadba2467c4c0d297d3)
Included flask.h in sidtab.h and policydb.c.
Change-Id: Ie12a7095fba32bb6eaee5d959f98ab6f9bc01b9a
Bug: 140252993
Signed-off-by: Jeff Vander Stoep <jeffv@google.com>