msghdr: print SCM_TIMESTAMP* control messages

msghdr: print SCM_TIMESTAMP* control messages

Add support for printing control messages containing software and/or
hardware timestamps, which are typically enabled by NTP and PTP
implementations.

* print_timeval.c (print_struct_timeval_data_size): New mpers printer.
* print_timespec.c (print_struct_timespec_data_size,
print_struct_timespec_array_data_size): Likewise.
* msghdr.c (print_scm_timestamp, print_scm_timestampns,
print_scm_timestamping): New functions.
(cmsg_socket_printers): Add them.
* NEWS: Mention this change.
* tests/msg_control.c: Include "xlat.h" and "xlat/scmvals.h".
(test_scm_timestamp, test_scm_timestampns, test_scm_timestamping):
New functions.
(test_sol_socket): Use them.

Signed-off-by: Miroslav Lichvar <mlichvar@redhat.com>
Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>