container: sched policy string representation

This also uses priority obtained via sched_get_priority_min, and improves bounds checking.

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

internal/pkg/exec.go

@@ -40,7 +40,7 @@ type ExecPath struct {
 }
 
 // SchedPolicy is the [container] scheduling policy.
-var SchedPolicy int
+var SchedPolicy container.SchedPolicy
 
 // PromoteLayers returns artifacts with identical-by-content layers promoted to
 // the highest priority instance, as if mounted via [ExecPath].