core/service: rework ExecReload= + Type=notify-reload interaction, add ExecReloadPost=

When Type=notify-reload got introduced, it wasn't intended to be
mutually exclusive with ExecReload=. However, currently ExecReload=
is immediately forked off after the service main process is signaled,
leaving states in between essentially undefined. Given so broken
it is I doubt any sane user is using this setup, hence I took a stab
to rework everything:

1.  Extensions are refreshed (unchanged)
2.  ExecReload= is forked off without signaling the process
3a. If RELOADING=1 is sent during the ExecReload= invocation,
    we'd refrain from signaling the process again, instead
    just transition to SERVICE_RELOAD_NOTIFY directly and
    wait for READY=1
3b. If not, signal the process after ExecReload= finishes
    (from now on the same as Type=notify-reload w/o ExecReload=)
4.  To accomodate the use case of performing post-reload tasks,
    ExecReloadPost= is introduced which executes after READY=1

The new model greatly simplifies things, as no control processes
will be around in SERVICE_RELOAD_SIGNAL and SERVICE_RELOAD_NOTIFY
states.

See also: https://github.com/systemd/systemd/issues/37515#issuecomment-2891229652

src/basic/unit-def.c

@@ -224,10 +224,11 @@ static const char* const service_state_table[_SERVICE_STATE_MAX] = {
         [SERVICE_START_POST]                 = "start-post",
         [SERVICE_RUNNING]                    = "running",
         [SERVICE_EXITED]                     = "exited",
+        [SERVICE_REFRESH_EXTENSIONS]         = "refresh-extensions",
         [SERVICE_RELOAD]                     = "reload",
         [SERVICE_RELOAD_SIGNAL]              = "reload-signal",
         [SERVICE_RELOAD_NOTIFY]              = "reload-notify",
-        [SERVICE_REFRESH_EXTENSIONS]         = "refresh-extensions",
+        [SERVICE_RELOAD_POST]                = "reload-post",
         [SERVICE_STOP]                       = "stop",
         [SERVICE_STOP_WATCHDOG]              = "stop-watchdog",
         [SERVICE_STOP_SIGTERM]               = "stop-sigterm",

src/basic/unit-def.h

@@ -131,10 +131,11 @@ typedef enum ServiceState {
         SERVICE_START_POST,
         SERVICE_RUNNING,
         SERVICE_EXITED,                 /* Nothing is running anymore, but RemainAfterExit is true hence this is OK */
+        SERVICE_REFRESH_EXTENSIONS,     /* Refreshing extensions for a reload request */
         SERVICE_RELOAD,                 /* Reloading via ExecReload= */
         SERVICE_RELOAD_SIGNAL,          /* Reloading via SIGHUP requested */
         SERVICE_RELOAD_NOTIFY,          /* Waiting for READY=1 after RELOADING=1 notify */
-        SERVICE_REFRESH_EXTENSIONS,     /* Refreshing extensions for a reload request */
+        SERVICE_RELOAD_POST,
         SERVICE_MOUNTING,               /* Performing a live mount into the namespace of the service */
         SERVICE_STOP,                   /* No STOP_PRE state, instead just register multiple STOP executables */
         SERVICE_STOP_WATCHDOG,