[libattach] misleading error message when checking gid fails

[Florian Weimer]

* stuart nelson:

> The reason getegid() returns -1 is because it wasn't in my allowed
> syscalls list for seccomp, so EPERM (-1) was returned instead.

Surely that's a broken seccomp filter.  Such fundamental system calls
really cannot be filtered.

Thanks,
Florian