process-util: Add support SCHED_EXT scheduling policy

Allow CPUSchedulingPolicy to be set to "ext". SCHED_EXT is a new
scheduling policy in Linux v6.12 that allows processes to be scheduled
using custom BPF schedulers instead of the default in-kernel ones.

Selectively setting the SCHED_EXT policy is useful for systems running
in "partial mode" where not all processes are run using a custom
scheduler.

Fallback to SCHED_OTHER and print an error message for systems where
SCHED_EXT isn't available.

src/test/test-sched-prio.c

@@ -13,7 +13,7 @@
 int main(int argc, char *argv[]) {
         _cleanup_(rm_rf_physical_and_freep) char *runtime_dir = NULL;
         _cleanup_(manager_freep) Manager *m = NULL;
-        Unit *idle_ok, *idle_bad, *rr_ok, *rr_bad, *rr_sched;
+        Unit *idle_ok, *idle_bad, *rr_ok, *rr_bad, *rr_sched, *ext_ok;
         Service *ser;
         int r;
 
@@ -76,5 +76,11 @@ int main(int argc, char *argv[]) {
         assert_se(ser->exec_context.cpu_sched_policy == SCHED_RR);
         assert_se(ser->exec_context.cpu_sched_priority == 99);
 
+        /* load ext ok */
+        assert_se(manager_load_startable_unit_or_warn(m, "sched_ext_ok.service", NULL, &ext_ok) >= 0);
+        ser = SERVICE(ext_ok);
+        assert_se(ser->exec_context.cpu_sched_policy == SCHED_EXT);
+        assert_se(ser->exec_context.cpu_sched_priority == 0);
+
         return EXIT_SUCCESS;
 }