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/core/socket.c

@@ -2488,7 +2488,7 @@ static int socket_start(Unit *u) {
 
                 /* If the service is already active we cannot start the
                  * socket */
-                if (!IN_SET(service->state, SERVICE_DEAD, SERVICE_FAILED, SERVICE_AUTO_RESTART))
+                if (!IN_SET(service->state, SERVICE_DEAD, SERVICE_FAILED, SERVICE_DEAD_BEFORE_AUTO_RESTART, SERVICE_FAILED_BEFORE_AUTO_RESTART, SERVICE_AUTO_RESTART))
                         return log_unit_error_errno(u, SYNTHETIC_ERRNO(EBUSY), "Socket service %s already active, refusing.", UNIT(service)->id);
         }
 
@@ -3291,7 +3291,7 @@ static void socket_trigger_notify(Unit *u, Unit *other) {
                 return;
 
         if (IN_SET(SERVICE(other)->state,
-                   SERVICE_DEAD, SERVICE_FAILED,
+                   SERVICE_DEAD, SERVICE_DEAD_BEFORE_AUTO_RESTART, SERVICE_FAILED, SERVICE_FAILED_BEFORE_AUTO_RESTART,
                    SERVICE_FINAL_SIGTERM, SERVICE_FINAL_SIGKILL,
                    SERVICE_AUTO_RESTART))
                socket_enter_listening(s);