service: kill stubborn services, wait before restart

Add support for registering callbacks to be called after a given
timeout.

Use this to implement forceful termination of services that refuse to
terminate upon receiving a SIGTERM. If the service PID is not collected
within 3 seconds, send a SIGKILL to it.

Also, rework restarting of crashing services. When a service crashes for
the first time, try to restart it immediately. If it crashes again, wait
a few seconds before trying again. Just as before, if it continues to
crash, block the service.

svc.h

@@ -65,6 +65,7 @@ typedef enum {
 	SVC_BLOCK_CRASHING,
 	SVC_BLOCK_USER,
 	SVC_BLOCK_INETD_BUSY,
+	SVC_BLOCK_RESTARTING,
 } svc_block_t;
 
 #define FINIT_SHM_ID     0x494E4954  /* "INIT", see ascii(7) */
@@ -123,6 +124,11 @@ typedef struct svc {
 	int	       dynamic_stop;
 	int	       private;
 	svc_cmd_t    (*cb)(struct svc *svc, int event, void *event_arg);
+
+	/* Used to forcefully kill services that won't shutdown on
+	 * termination and to delay restarts of crashing services. */
+	uev_t          timer;
+	void           (*timer_cb)(struct svc *svc);
 } svc_t;
 
 typedef struct svc_map svc_map_t;