* src/gcrypt.h.in (GCRY_KDF_ONESTEP_KDF): New.
* cipher/kdf.c (onestep_kdf_open, onestep_kdf_compute): New.
(onestep_kdf_final): New.
(_gcry_kdf_open, _gcry_kdf_compute, _gcry_kdf_final): Add
GCRY_KDF_ONESTEP_KDF support.
* tests/t-kdf.c (check_onestep_kdf): Add the test.
(main): Call check_onestep_kdf.