Editorial changes and allow building with old libgcrypts.

Changed order of some conditional to make to put the special case into
the true branch. Indentation changes. Minor other changes to make the
ECC code more similar to the rest of our code.

It builds but many sefltests still fail. Need to fix that before
using it with an ECDH enabled libgcrypt.

2011-01-21 Werner Koch <wk@g10code.com>

• configure.ac: Need Libgcrypt 1.4.6 due to AESWRAP.

(HAVE_GCRY_PK_ECDH): Add new test.

[agent/]
2011-01-21 Werner Koch <wk@g10code.com>

• cvt-openpgp.c (GCRY_PK_ECDH) [!HAVE_GCRY_PK_ECDH]: New.

[include/]
2011-01-21 Werner Koch <wk@g10code.com>

• cipher.h (GCRY_PK_USAGE_CERT): Remove compatibility macros

because we now require libgcrypt 1.4.6.
(GCRY_PK_ECDH): Add replacement.