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/dbus-execute.c

@@ -3167,7 +3167,7 @@ int bus_exec_context_set_transient_property(
                                 return r;
 
                         c->cpu_sched_policy = q;
-                        c->cpu_sched_priority = CLAMP(c->cpu_sched_priority, sched_get_priority_min(q), sched_get_priority_max(q));
+                        c->cpu_sched_priority = CLAMP(c->cpu_sched_priority, sched_get_priority_min_safe(q), sched_get_priority_max_safe(q));
                         c->cpu_sched_set = true;
 
                         unit_write_settingf(u, flags, name, "CPUSchedulingPolicy=%s", s);