Buxton - list_keys

This changeset implements `buxton_{direct,client}_list_keys`

Confirmed as working via --direct and wire-protocol, also run heavily under valgrind and gdb for two days, as made evident by the memleak fixes included.

Once integrated, we need to discuss smack access for these changes, notably the daemon should create a new BuxtonArray, and only add the results that the client is entitled to see (read permission). Lets get this in first before planning that bit though :)