bpftool: make key and value optional in update command

Bpftool expects both key and value for 'update' operations. For some
map types, key should not be specified. Support updating those map types.

Before:
bpftool map create /sys/fs/bpf/q type queue value 4 entries 10 name q
bpftool map update pinned /sys/fs/bpf/q value 0 1 2 3
Error: did not find key

After:
bpftool map create /sys/fs/bpf/q type queue value 4 entries 10 name q
bpftool map update pinned /sys/fs/bpf/q value 0 1 2 3

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>