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/load-fragment.c

@@ -1539,9 +1539,12 @@ int config_parse_exec_cpu_sched_policy(
                 return 0;
         }
 
+        if (!sched_policy_supported(x))
+                log_syntax(unit, LOG_WARNING, filename, line, x, "Unsupported CPU scheduling policy: %s", rvalue);
+
         c->cpu_sched_policy = x;
         /* Moving to or from real-time policy? We need to adjust the priority */
-        c->cpu_sched_priority = CLAMP(c->cpu_sched_priority, sched_get_priority_min(x), sched_get_priority_max(x));
+        c->cpu_sched_priority = CLAMP(c->cpu_sched_priority, sched_get_priority_min_safe(x), sched_get_priority_max_safe(x));
         c->cpu_sched_set = true;
 
         return 0;