implement Signed-By without using gpg for verification

implement Signed-By without using gpg for verification

The previous commit returns to the possibility of using just gpgv for
verification proposes. There is one problem through: We can't enforce a
specific keyid without using gpg, but our acquire method can as it
parses gpgv output anyway, so it can deal with good signatures from not
expected signatures and treats them as unknown keys instead.

Git-Dch: Ignore