tools/bpftool: Add `link detach` subcommand

Add ability to force-detach BPF link. Also add missing error message, if
specified link ID is wrong.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20200731182830.286260-5-andriin@fb.com