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/core/exec-invoke.c

@@ -5473,8 +5473,12 @@ int exec_invoke(
 
                 r = sched_setattr(/* pid= */ 0, &attr, /* flags= */ 0);
                 if (r < 0) {
-                        *exit_status = EXIT_SETSCHEDULER;
-                        return log_error_errno(errno, "Failed to set up CPU scheduling: %m");
+                        if (errno != EINVAL || sched_policy_supported(attr.sched_policy)) {
+                                *exit_status = EXIT_SETSCHEDULER;
+                                return log_error_errno(errno, "Failed to set up CPU scheduling: %m");
+                        }
+
+                        log_warning_errno(errno, "CPU scheduling policy %u is not supported, ignoring: %m", attr.sched_policy);
                 }
         }