s4-ldb: Add support for binary blobs in DNs

s4-ldb: Add support for binary blobs in DNs

AD has the concept of a DN prefixed with B:NN:XXXXXX: that contains a
binary blob. We need to support those in order to give correctly
formatted binary blobs for things like wellKnownObjects

This implementation is not ideal, as it allows for binary blobs on all
DNs, whereas it should only allow them on those with a syntax of
2.5.5.7. We should clean this up in the future, but meanwhile this
implementation at least gets us a working DC join of w2k8 to s4.

This patch also uses a static function for marking DNs as invalid,
which is very useful when debugging this code, as you can break on it
in gdb.