Support creation of ECDH enveloped data object (part 2 of 2)

* src/cms.c (build_enveloped_data_header): Write out ECDH info.

This finishes the building of CMS EnvelopeData for ECDH. Tested with
one and two keys (also mixed RSA and ECDH) but only only with
t-cms-parser and GnuPG master and not against any other
implementation.

• GnuPG-bug-id: T4920
• Signed-off-by: Werner Koch <wk@gnupg.org>