bpftool: add peek command

This is intended to be used with queues and stacks and be more
user-friendly than 'lookup' without key/value.

Example:
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
bpftool map peek pinned /sys/fs/bpf/q
value: 00 01 02 03

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