core: Check unit start rate limiting earlier

Fixes #17433. Currently, if any of the validations we do before we
check start rate limiting fail, we can still enter a busy loop as
no rate limiting gets applied. A common occurence of this scenario
is path units triggering a service that fails a condition check.

To fix the issue, we simply move up start rate limiting checks to
be the first thing we do when starting a unit. To achieve this,
we add a new method to the unit vtable and implement it for the
relevant unit types so that we can do the start rate limit checks
earlier on.

src/core/path.c

@@ -590,12 +590,6 @@ static int path_start(Unit *u) {
         if (r < 0)
                 return r;
 
-        r = unit_test_start_limit(u);
-        if (r < 0) {
-                path_enter_dead(p, PATH_FAILURE_START_LIMIT_HIT);
-                return r;
-        }
-
         r = unit_acquire_invocation_id(u);
         if (r < 0)
                 return r;
@@ -812,6 +806,21 @@ static void path_reset_failed(Unit *u) {
         p->result = PATH_SUCCESS;
 }
 
+static int path_test_start_limit(Unit *u) {
+        Path *p = PATH(u);
+        int r;
+
+        assert(p);
+
+        r = unit_test_start_limit(u);
+        if (r < 0) {
+                path_enter_dead(p, PATH_FAILURE_START_LIMIT_HIT);
+                return r;
+        }
+
+        return 0;
+}
+
 static const char* const path_type_table[_PATH_TYPE_MAX] = {
         [PATH_EXISTS]              = "PathExists",
         [PATH_EXISTS_GLOB]         = "PathExistsGlob",
@@ -866,4 +875,6 @@ const UnitVTable path_vtable = {
         .reset_failed = path_reset_failed,
 
         .bus_set_property = bus_path_set_property,
+
+        .test_start_limit = path_test_start_limit,
 };