OTHER: Add void arg to param-less funcs.
OTHER: Remove unused _oplist_is_empty function.
OTHER: Add an order-builder to the s4_resultset_sort API.
BUG(2555): Update g_(thread|mutex|cond)_ calls to GLib 2.32+ API.
BUG(2555): Remove a lingering g_thread_init call in the s4 cli.
OTHER: Update test tools to match XMMS2.