GST-based backend does not require opus at compile time

git-svn-id: https://gnunet.org/svn/gnunet@31982 140774ce-b5e7-0310-ab8b-a85725594a96
- scalarproduct element container was 1 byte too large
- added logics to compute the maximum supported number of paillier hom.ops for a ciphertext
- updated comments

git-svn-id: https://gnunet.org/svn/gnunet@31983 140774ce-b5e7-0310-ab8b-a85725594a96
- added logics for homomorphic operation in paillier
- adjusted headers

git-svn-id: https://gnunet.org/svn/gnunet@31984 140774ce-b5e7-0310-ab8b-a85725594a96