common,agent: Factor out KEM functions into common/kem.c.
* common/util.h (compute_kmac256): Remove. (gnupg_ecc_kem_kdf, gnupg_kem_combiner): New. * common/kmac.c (compute_kmac256): Don't expose. (gnupg_ecc_kem_kdf, gnupg_kem_combiner): New. * agent/pkdecrypt.c (agent_hybrid_pgp_kem_decrypt): Use gnupg_ecc_kem_kdf and gnupg_kem_combiner.
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>