core: increment start limit counter only when we can start the unit

Otherwise, e.g. requesting to start a unit that is under stopping may
enter the failed state.

This makes
- rename .can_start() -> .test_startable(), and make it allow to return
  boolean and refuse to start units when it returns false,
- refuse earlier to start units that are in the deactivating state, so
  several redundant conditions in .start() can be dropped,
- move checks for unit states mapped to UNIT_ACTIVATING from .start() to
  .test_startable().

Fixes #39247.

src/core/timer.c

@@ -906,7 +906,7 @@ static int timer_can_clean(Unit *u, ExecCleanMask *ret) {
         return 0;
 }
 
-static int timer_can_start(Unit *u) {
+static int timer_test_startable(Unit *u) {
         Timer *t = ASSERT_PTR(TIMER(u));
         int r;
 
@@ -916,7 +916,7 @@ static int timer_can_start(Unit *u) {
                 return r;
         }
 
-        return 1;
+        return true;
 }
 
 static void activation_details_timer_serialize(const ActivationDetails *details, FILE *f) {
@@ -1093,7 +1093,7 @@ const UnitVTable timer_vtable = {
 
         .bus_set_property = bus_timer_set_property,
 
-        .can_start = timer_can_start,
+        .test_startable = timer_test_startable,
 };
 
 const ActivationDetailsVTable activation_details_timer_vtable = {