Add new hash chain entries for mandatory life signs

[frankbraun]

Add two new hash chain entry types:

lifesn
nolife

and corresponding codechain commands to enable and disable mandatory lifesign signals by the developers.

If lifesigns are enabled, the developers have to sign the hash chain every user defined interval, even if no code changes occurred.

This gives cryptographic proof that m of the n developers are still able to use their keys.