Reverted the API to the old one.

Storing thy curve name into an MPI and dropping all the other
parameters breaks the ABI. Further it as not anymore possible to
specify arbitrary curves. Thus I had to revert it to the old ABI.

Another advantage is that the required ECDH changes are now minimal.
The disadvantage is that we need to adjust the GnuPG code and do the
curve selection and translation to DER encoded OIDs itself.