encoding (internal): To support signed interests, in ndn_encodeBinaryXmlName, return signedPortionBeginOffset and signedPortionEndOffset, and return these from ndn_encodeBinaryXmlInterest.
encoding (internal): To support signed interests, in ndn_encodeTlvName, return signedPortionBeginOffset and signedPortionEndOffset, and return these from ndn_encodeTlvInterest.
encoding (internal): To support signed interests, in WireFormat::encodeInterest, return signedPortionBeginOffset and signedPortionEndOffset.
Interest (internal): To support signed interests, make wireEncode return a SignedBlob with the offsets of the signed portion of the name.