Refactor, replace BSD queue.h LIST_ macros with homegrown version

This patch replaces the BSD doubly linked list implementation with a
simplified homegrown version.  This not only reduces the dependency on
an external "library", but also greatly simplifies licensing and gives
us a clean name space that does not intrude on users who want to use
their own version of queue.h in their application that also includes
the libuEv header file.

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

src/cron.c

@@ -112,9 +112,6 @@ int uev_cron_set(uev_t *w, time_t when, time_t interval)
 		if (!when && !interval)
 			return 0;
 
-		/* Remove from internal list */
-		LIST_REMOVE(w, link);
-
 		if (uev_cron_init(w->ctx, w, (uev_cb_t *)w->cb, w->arg, when, interval))
 			return -1;
 	}