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

lib/cf-socket.h

@@ -81,9 +81,9 @@ int Curl_socket_close(struct Curl_easy *data, struct connectdata *conn,
    Buffer Size
 
 */
-void Curl_sndbufset(curl_socket_t sockfd);
+void Curl_sndbuf_init(curl_socket_t sockfd);
 #else
-#define Curl_sndbufset(y) Curl_nop_stmt
+#define Curl_sndbuf_init(y) Curl_nop_stmt
 #endif
 
 /**