cf-socket: improve SO_SNDBUF update for Winsock

cf-socket: improve SO_SNDBUF update for Winsock

- Rename: Curl_sndbufset => Curl_sndbuf_init

- Rename: win_update_buffer_size => win_update_sndbuf_size

- Save the last set SO_SNDBUF size to compare against so that we can
  avoid setsockopt calls every second.

This is a follow-up to 0b520e12 which moved the SO_SNDBUF update check
into cf-socket. This change improves it further by making the function
names easier to understand and reducing the amount of setsockopt calls.

Closes https://github.com/curl/curl/pull/13827