Support listing BPF iterators

[qmonnet]

bpftool iter only support the pin subcommands. It begs for a list subcommands. Not sure if this is completely necessary though, given that we can already see the iterators from bpftool prog. At least we've got the code for listing them already: if we want to add a bpftool iter list we can probably just reuse the code from bpftool prog list, filtering on program type.