Update portable memmem()

[Damien Miller]

On Fri, 7 Aug 2020, Mathias wrote:

> Hello.
> I have noticed that openbsd-compat/memmem.c from OpenSSH portable is
> still using an old version of OpenBSD's memmem(). Recently OpenBSD's
> received a big update to increase performance. It would make sense for
> OpenSSH to update its version as well.
> Link: https://cvsweb.openbsd.org/src/lib/libc/string/memmem.c?rev=1.5

I have synced it. It probably doesn't matter much because memmem() is
used only in one place that is not performance-critical and with short
strings.

-d