system/acl: wrap libacl errors in PathError

This helps determine which libacl function the errno came from.

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

system/acl/acl.go

@@ -29,8 +29,5 @@ func Update(name string, uid int, perms ...Perm) error {
 		(*C.acl_perm_t)(p),
 		C.size_t(len(perms)),
 	)
-	if r == 0 {
-		return nil
-	}
-	return err
+	return newAclPathError(name, int(r), err)
 }