gnutls_transport_set_lowat

[Eli Zaretskii]

We use this function in gnutls.c only on Windows.  Latest versions of
GnuTLS removed this, which currently precludes Emacs from working with
GnuTLS 3.x.

>From the comments in gnutls.c and also from various Google hits, it
seems that we should simply not call this functions in GnuTLS 3.x, as
the low-water functionality is now disabled by default.  It is easy to
make a change that would do that, but would someone who knows their
ways around GnuTLS please confirm that this is TRT?