sched: always return non-zero timeout ID

Timeout ID of zero can be now safely used to indicate that the timer is
not running. Remove the extra timer_running variables that were
necessary to track that.

sched.h

@@ -29,6 +29,7 @@
 
 #include "sysincl.h"
 
+/* Type for timeout IDs, valid IDs are always greater than zero */
 typedef unsigned long SCH_TimeoutID;
 
 typedef enum {