container: expose priority and SCHED_OTHER policy

The more explicit API removes the arbitrary limit preventing use of SCHED_OTHER (referred to as SCHED_NORMAL in the kernel). This change also exposes priority value to set.

Signed-off-by: Ophestra <cat@gensokyo.uk>

test/test.py

@@ -213,6 +213,9 @@ if sched_unset != 0:
 sched_idle = int(machine.succeed("sudo -u alice -i hakurei -v run --sched=idle cat /proc/self/sched | grep '^policy' | tr -d ' ' | cut -d ':' -f 2"))
 if sched_idle != 5:
     raise Exception(f"unexpected idle policy: {sched_idle}")
+sched_rr = int(machine.succeed("sudo -u alice -i hakurei -v run --sched=rr cat /proc/self/sched | grep '^policy' | tr -d ' ' | cut -d ':' -f 2"))
+if sched_rr != 2:
+    raise Exception(f"unexpected round-robin policy: {sched_idle}")
 
 # Start app (foot) with Wayland enablement:
 swaymsg("exec ne-foot")