Change to generate compliant keys for compact representation.

[Andrey Jivsov]

Greetings. I updated cipher/ecc.c in libgcrypt so that GnuPG generates 
only compliant keys that would allow the easiest compact representation, 
according to http://tools.ietf.org/html/draft-jivsov-ecc-compact.

AKAIK, there is no reason to not to take this change. It doesn't affect 
anything else right now, doesn't change anything about compact 
representation. It only enables the easiest and the most efficient ECC 
point representation that can be implemented in the future.

I am planning to write a spec on how to do compact point representation 
in OpenPGP. It's mostly needed for encrypted messages (sent to a large 
number of recipients), but is also valuable for anybody who cares about 
ultra-compact key size. OpenPGP offers very lean encoding format and so 
that if one's goal is to have the smallest public key, OpenPGP encoding 
is probably the best choice.

Thank you.