DNS: signed head should contain last signed hash chain line number

[frankbraun]

To be able to move secpkg to multiple DNS URLs the signed head should contain the last signed hash chain line number.

This would allow to easily determine which DNS URL announces the most current head.

[frankbraun]

secpkg can now deal with version 1 and version 2 entries, but we wait a while for all clients to updates until we actually write version 2 TXT records.