Fix Coverity CID CID 56281: Resource leak (RESOURCE_LEAK)

Initial support for unloading plugins.  Currently we only save dlopen()
handle.  The plugin_unregister() function itself is never called atm.
This should at the very least make Coverity happy.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

plugin.h

@@ -79,6 +79,9 @@ typedef struct plugin {
 	/* BSD sys/queue.h linked list node. */
 	TAILQ_ENTRY(plugin) link;
 
+	/* Handle to be sent to dlclose() when plugins is unloaded. */
+	void *handle;
+
 	/* Event loop handler, used internally by Finit */
 	uev_t watcher;