netlink_unix_diag: implement UDIAG_SHOW_UID/UNIX_DIAG_UID decoding

netlink_unix_diag: implement UDIAG_SHOW_UID/UNIX_DIAG_UID decoding

... introduced by Linux kernel commit v5.3-rc1~140^2~467.

* linux/unix_diag.h (UDIAG_SHOW_UID, UNIX_DIAG_FIRST_UNUSED): New macro
constants.
(UNIX_DIAG_UID): New enum constant.
* xlat/unix_diag_attrs.in: Add UNIX_DIAG_UID.
* xlat/unix_diag_show.in: Add UDIAG_SHOW_UID.
* netlink_unix_diag.c (unix_diag_msg_nla_decoders): Handle
UNIX_DIAG_UID.
* NEWS: Mention this.
* tests/nlattr.c (test_nlattr, test_nla_type): Replace
UNIX_DIAG_SHUTDOWN + 1 with UNIX_DIAG_FIRST_UNUSED.