Implement decoding of BPF_*_GET_*_ID commands of bpf syscall

Implement decoding of BPF_*_GET_*_ID commands of bpf syscall

* configure.ac: Check for union bpf_attr.next_id.
* bpf.c (decode_BPF_PROG_GET_NEXT_ID, decode_BPF_PROG_GET_FD_BY_ID,
* decode_BPF_MAP_GET_FD_BY_ID): New functions.
(decode_BPF_MAP_GET_NEXT_ID): New macro.
(SYS_FUNC(bpf)) <bpf_cmd_decoders>: Use them.
* NEWS: Mention this.
* tests/bpf.c: Add macro guard for BPF_*_GET_*_ID decoder tests.
[HAVE_UNION_BPF_ATTR_NEXT_ID] (init_BPF_PROG_GET_NEXT_ID_first,
print_BPF_PROG_GET_NEXT_ID_first, init_BPF_PROG_GET_NEXT_ID_attr,
print_BPF_PROG_GET_NEXT_ID_attr, print_BPF_PROG_GET_FD_BY_ID_first,
print_BPF_PROG_GET_FD_BY_ID_attr, print_BPF_MAP_GET_NEXT_ID_first,
print_BPF_MAP_GET_NEXT_ID_attr): New functions.
(init_BPF_MAP_GET_NEXT_ID_first, print_BPF_MAP_GET_NEXT_ID_first,
init_BPF_MAP_GET_NEXT_ID_attr, print_BPF_MAP_GET_NEXT_ID_attr,
init_BPF_PROG_GET_FD_BY_ID_first, init_BPF_PROG_GET_FD_BY_ID_attr,
init_BPF_MAP_GET_FD_BY_ID_first, init_BPF_MAP_GET_FD_BY_ID_attr):
New macros.
(main) [HAVE_UNION_BPF_ATTR_NEXT_ID]: Use them.