keyctl: add support for KDF parameters decoding in KEYCTL_DH_COMPUTE

keyctl: add support for KDF parameters decoding in KEYCTL_DH_COMPUTE

* fetch_struct_keyctl_kdf_params.c: New file.
* keyctl_kdf_params.h: Likewise.
* Makefile.am (strace_SOURCES): Add them.
* configure.ac: Add check for struct keyctl_kdf_params presence in
<linux/keyctl.h>.
* defs.h (struct strace_keyctl_kdf_params): Add forward declaration.
* keyctl.c (keyctl_dh_compute): Add new parameter kdf_addr, print it
on exiting.
(SYS_FUNC(keyctl)) <case KEYCTL_DH_COMPUTE>: Pass arg5 to
keyctl_dh_compute.
* tests/keyctl.c: Include assert.h.
(struct keyctl_kdf_params) [!HAVE_STRUCT_KEYCTL_KDF_PARAMS]: New
definition.
(STR32): New definition, copied from ioctl_dm.c.
(append_str, kckdfp_to_str): New functions.
(main): Update expected output, add checks for struct keyctl_kdf_params
decoding.