cli: Implement list_keys

See commit message for gory details.
Note after this becomes merged we'll need to export BuxtonList (seems most appropriate for export instead of BuxtonArray) and for the public API part we should actually switch to using BuxtonKey's in our BuxtonList, not BuxtonStrings. Buxton buxton buxton. (Sorry, name is everywhere :P)

So, there is a method in place to re-extract the group name in gdbm, we're just missing the SMACK labels. I propose we modify the method to take the current smack label (via api, from buxtond, etc) to actually recreate a BuxtonKey listing. Seems way more appropriate (Also we can check then if they have the right label in the listing :))

Valgrinded in --direct and via wire, can't see any leaks