let generate_releases be able to use multiple keys for a signature

let generate_releases be able to use multiple keys for a signature

without having two signature blocks, which apt / co arent really checking.
now they get them presented in one block, so they might actually do so.

only really interesting whenever we do a key rollover of the ftpmaster key,
as we dont have the stable key available. pity, or it would work there to
to make this kind of "merged" signature.