sm: Support decryption of ECDH data.

* sm/decrypt.c: Include tlv.h.
(string_from_gcry_buffer): New.
(hash_ecc_cms_shared_info): New.
(ecdh_decrypt): New.
(prepare_decryption): Support ECDH.  Add arg pk_algo.
(gpgsm_decrypt): Lift some variables from an inner code block.

Note: This has only been tested with a single messages created by
OpenSSL and taken from the Mozilla bug tracker. In particular the
code to included UserKeyingMaterial (ukm) has not been tested.

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