During implementation MN EHF several improvements came up for DIP-0023.

• renamed version to versionBit in MnEhf Signal to avoid confusion. version inside signal doesn't mean a version of message. It means "number of version bit" for chain params (BIP9). This one is the most important changes from these 3.
• for Tx "Mn EHF" unlikely we will have more than 255 version ever. That's very minor improvement because this type of transaction is rare, but I see no good reason to have 2 bytes for version.
• size of "MnEHF signal" is known and it is exactly 129 bytes. The length is not "various" depending on content. Fixed this one in DIP too.