Fixed the ECC interface to Libgcrypt to be ABI compatible with the previous version.

Quite some changes were needed but in the end we have less code than
before. Instead of trying to do everything with MPIs and pass them
back and forth between Libgcrypt and GnuPG, we know use the
S-expression based interface and make heavy use of our opaque MPI
feature.

Encryption, decryption, signing and verification work with
self-generared keys.

Import and export does not yet work; thus it was not possible to check
the test keys at https://sites.google.com/site/brainhub/pgpecckeys .