core: let user define start-/stop-timeout behaviour

The usual behaviour when a timeout expires is to terminate/kill the
service. This is what user usually want in production systems. To debug
services that fail to start/stop (especially sporadic failures) it
might be necessary to trigger the watchdog machinery and write core
dumps, though. Likewise, it is usually just a waste of time to
gracefully stop a stuck service. Instead it might save time to go
directly into kill mode.

This commit adds two new options to services: TimeoutStartFailureMode=
and TimeoutStopFailureMode=. Both take the same values and tweak the
behavior of systemd when a start/stop timeout expires:

 * 'terminate': is the default behaviour as it has always been,
 * 'abort': triggers the watchdog machinery and will send SIGABRT
   (unless WatchdogSignal was changed) and
 * 'kill' will directly send SIGKILL.

To handle the stop failure mode in stop-post state too a new
final-watchdog state needs to be introduced.

src/core/service.h

@@ -74,6 +74,14 @@ typedef enum ServiceResult {
         _SERVICE_RESULT_INVALID = -1
 } ServiceResult;
 
+typedef enum ServiceTimeoutFailureMode {
+        SERVICE_TIMEOUT_TERMINATE,
+        SERVICE_TIMEOUT_ABORT,
+        SERVICE_TIMEOUT_KILL,
+        _SERVICE_TIMEOUT_FAILURE_MODE_MAX,
+        _SERVICE_TIMEOUT_FAILURE_MODE_INVALID = -1
+} ServiceTimeoutFailureMode;
+
 struct ServiceFDStore {
         Service *service;
 
@@ -103,6 +111,8 @@ struct Service {
         usec_t timeout_abort_usec;
         bool timeout_abort_set;
         usec_t runtime_max_usec;
+        ServiceTimeoutFailureMode timeout_start_failure_mode;
+        ServiceTimeoutFailureMode timeout_stop_failure_mode;
 
         dual_timestamp watchdog_timestamp;
         usec_t watchdog_usec;            /* the requested watchdog timeout in the unit file */
@@ -228,6 +238,9 @@ NotifyState notify_state_from_string(const char *s) _pure_;
 const char* service_result_to_string(ServiceResult i) _const_;
 ServiceResult service_result_from_string(const char *s) _pure_;
 
+const char* service_timeout_failure_mode_to_string(ServiceTimeoutFailureMode i) _const_;
+ServiceTimeoutFailureMode service_timeout_failure_mode_from_string(const char *s) _pure_;
+
 DEFINE_CAST(SERVICE, Service);
 
 #define STATUS_TEXT_MAX (16U*1024U)