client: finish set_label support

The set_label functionality was missing "set-label-on-key" support; this
only worked for those calling buxton_direct_set_label().

Finish this work by giving non-direct root clients the ability to set
labels on keys as well as groups.

The buxtonctl client already documents that setting labels on keys is
supported, but until now, it would fail silently unless running in
direct mode.