pid1: introduce new SERVICE_{DEAD|FAILED}_BEFORE_AUTO_RESTART service substates

When a service deactivates and is then automatically restarted via
Restart= we currently quickly transition through
SERVICE_DEAD/SERVICE_FAILED. Which is weird given it's not the
normal ("permanent") dead/failed state, but a transitory one we
immediately leave from again. We do this so that software that looks for
failures/successes can take notice, even if we restart as a consequence
of the deactivation.

Let's clean this up a bit: let's introduce two new states:
SERVICE_DEAD_BEFORE_AUTO_RESTART and SERVICE_FAILED_BEFORE_AUTO_RESTART
that are used for the transitory states. Both the SERVICE_DEAD and
SERVICE_DEAD_BEFORE_AUTO_RESTART will map to the high-level
UNIT_INACTIVE state though. (and similar for the respective failed
states). This means the high-level state machine won't change by this,
only the low-level one.

This clearly seperates the substates, which makes the state engine
cleaner, and allows clients to follow precisely whether we are in a
transitory dead/failed state, or a permanent one, by looking at the
service substate. Moreover it allows us to remove the 'n_keep_fd_store'
which so far we used to ensure the fdstore was not released during this
transitory dead/failed state but only during the permanent one. Since we
can now distinguish these states properly we can just use that.

This has been bugging me for a while. Let's clean this up.

Note that the unit restart logic is already nicely covered in the
testsiute, hence this adds no new tests for that.

And yes, this could be considered a compat break, but sofar we took the
liberty to make changes to the low-level state machine (i.e. SERVICE_xyz
states, sometimes called "substates") without considering this a bad
breakage – the high-level state machine (i.e.  UNIT_xyz states) should
be considered API that cannot be changed.

src/basic/unit-def.c

@@ -180,27 +180,29 @@ static const char* const scope_state_table[_SCOPE_STATE_MAX] = {
 DEFINE_STRING_TABLE_LOOKUP(scope_state, ScopeState);
 
 static const char* const service_state_table[_SERVICE_STATE_MAX] = {
-        [SERVICE_DEAD]           = "dead",
-        [SERVICE_CONDITION]      = "condition",
-        [SERVICE_START_PRE]      = "start-pre",
-        [SERVICE_START]          = "start",
-        [SERVICE_START_POST]     = "start-post",
-        [SERVICE_RUNNING]        = "running",
-        [SERVICE_EXITED]         = "exited",
-        [SERVICE_RELOAD]         = "reload",
-        [SERVICE_RELOAD_SIGNAL]  = "reload-signal",
-        [SERVICE_RELOAD_NOTIFY]  = "reload-notify",
-        [SERVICE_STOP]           = "stop",
-        [SERVICE_STOP_WATCHDOG]  = "stop-watchdog",
-        [SERVICE_STOP_SIGTERM]   = "stop-sigterm",
-        [SERVICE_STOP_SIGKILL]   = "stop-sigkill",
-        [SERVICE_STOP_POST]      = "stop-post",
-        [SERVICE_FINAL_WATCHDOG] = "final-watchdog",
-        [SERVICE_FINAL_SIGTERM]  = "final-sigterm",
-        [SERVICE_FINAL_SIGKILL]  = "final-sigkill",
-        [SERVICE_FAILED]         = "failed",
-        [SERVICE_AUTO_RESTART]   = "auto-restart",
-        [SERVICE_CLEANING]       = "cleaning",
+        [SERVICE_DEAD]                       = "dead",
+        [SERVICE_CONDITION]                  = "condition",
+        [SERVICE_START_PRE]                  = "start-pre",
+        [SERVICE_START]                      = "start",
+        [SERVICE_START_POST]                 = "start-post",
+        [SERVICE_RUNNING]                    = "running",
+        [SERVICE_EXITED]                     = "exited",
+        [SERVICE_RELOAD]                     = "reload",
+        [SERVICE_RELOAD_SIGNAL]              = "reload-signal",
+        [SERVICE_RELOAD_NOTIFY]              = "reload-notify",
+        [SERVICE_STOP]                       = "stop",
+        [SERVICE_STOP_WATCHDOG]              = "stop-watchdog",
+        [SERVICE_STOP_SIGTERM]               = "stop-sigterm",
+        [SERVICE_STOP_SIGKILL]               = "stop-sigkill",
+        [SERVICE_STOP_POST]                  = "stop-post",
+        [SERVICE_FINAL_WATCHDOG]             = "final-watchdog",
+        [SERVICE_FINAL_SIGTERM]              = "final-sigterm",
+        [SERVICE_FINAL_SIGKILL]              = "final-sigkill",
+        [SERVICE_FAILED]                     = "failed",
+        [SERVICE_DEAD_BEFORE_AUTO_RESTART]   = "dead-before-auto-restart",
+        [SERVICE_FAILED_BEFORE_AUTO_RESTART] = "failed-before-auto-restart",
+        [SERVICE_AUTO_RESTART]               = "auto-restart",
+        [SERVICE_CLEANING]                   = "cleaning",
 };
 
 DEFINE_STRING_TABLE_LOOKUP(service_state, ServiceState);

src/basic/unit-def.h

@@ -144,6 +144,8 @@ typedef enum ServiceState {
         SERVICE_FINAL_SIGTERM,     /* In case the STOP_POST executable hangs, we shoot that down, too */
         SERVICE_FINAL_SIGKILL,
         SERVICE_FAILED,
+        SERVICE_DEAD_BEFORE_AUTO_RESTART,
+        SERVICE_FAILED_BEFORE_AUTO_RESTART,
         SERVICE_AUTO_RESTART,
         SERVICE_CLEANING,
         _SERVICE_STATE_MAX,