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.

test/test-sched-prio/sched_ext_ok.service

@@ -0,0 +1,8 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+[Unit]
+Description=Sched ext with prio 0
+
+[Service]
+ExecStart=true
+CPUSchedulingPriority=0
+CPUSchedulingPolicy=ext