libbpf: Add low-level APIs for new bpf_link commands

Add low-level API calls for bpf_link_get_next_id() and
bpf_link_get_fd_by_id().

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200429001614.1544-6-andriin@fb.com