Also: add support for ignoring null enctype / zero-length keys,
which *can* be found in MIT DB entries created in pre-historic
times.
Also: make the mitdb HDB backend more elegant (e.g., use the ASN.1
compiler's generated sequence/array utility functions.
Also: add a utility function needed for kadm5 kvno change
improvements and make kadmin's mod --kvno work correctly and
naturally.
Signed-off-by: Love Hornquist Astrand <lha@h5l.org>