Make dh_key_info encoder and decoder symmetric

Make dh_key_info encoder and decoder symmetric

The dh_key_info encoder expects subjectPublicKey to contain the
contents of a bit string, but the decoder outputs the DER encoding of
the bit string including tag.  The PKINIT client code expects this, so
everything works, but the encoder and decoder should be symmetric.
Change the decoder to process the bit string (adding a bit string
decoding primitive) and modify the PKINIT client code to expect only
the bit string contents.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@25626 dc483132-0cff-0310-8789-dd5450dbe970