sm: Support encryption using ECDH keys.

* sm/decrypt.c (hash_ecc_cms_shared_info): Make global.
* sm/encrypt.c (ecdh_encrypt): New.
(encrypt_dek): Add arg PK_ALGO and support ECDH.
(gpgsm_encrypt): Pass PK_ALGO.

Note: This has only been tested with a messages created and decrypted
by GnuPG.

• GnuPG-bug-id: T4098
• Signed-off-by: Werner Koch <wk@gnupg.org>