winsock: move SO_SNDBUF update into cf-socket

winsock: move SO_SNDBUF update into cf-socket

- Move the code that updates the SO_SNDBUF size for Windows to
  cf_socket_send.

Prior to this change the code was in readwrite_upload but the socket
filter is the more appropriate place because it applies to all sends.

Background:

For Windows users SO_SNDBUF (the total per-socket buffer size reserved
by Winsock for sends) is updated dynamically by libcurl during the
transfer. This is because Windows does not do it automatically for
non-blocking sockets and without it the performance of large transfers
may suffer.

Closes https://github.com/curl/curl/pull/13763