PR go/86331
    os: check return value as well as error from waitid
    
    https://gcc.gnu.org/PR86331 indicates that if a signal handler runs it
    is possible for syscall.Syscall6 to return a non-zero errno value even
    if no error occurs. That is a problem in general, but this fix will
    let us work around the general problem for the specific case of
    calling waitid.
    
    Reviewed-on: https://go-review.googlesource.com/121595

From-SVN: r262313