s3-pdb: Break SECRETS3 dependency on PDB.

s3-pdb: Break SECRETS3 dependency on PDB.

This is causing circular depdnendcies that bring libpdb in all code and this is
BAD.

This change 'protects' the sid and guid of the domain by adding a special key
that makes them effectively read only.

Limit this temporarily to the samba 4 build, once it gets some good testing the
samba4 ifdefs can be dropped.

fix pdb dependencies

Signed-off-by: Andreas Schneider <asn@samba.org>