setuid(2) differs from the OpenBSD setuid(2) in that -EPERM is
returned by the syscall even if the euid of the process matches the
uid passed to it.

Either I am non compos or the thing is very wrong. The docs
(man-pages-1.35) say

ERRORS
       EPERM  The  user  is  not the super-user, and uid does not
              match the effective or saved user ID of the calling
              process.

The following untested patch changes the kernel to match the
documentated behaviour.