sendf: move Curl_sendf to dict.c and make it static

... as the only remaining user of that function. Also fix gopher.c to
instead use Curl_write()

Closes #6020

lib/gopher.c

@@ -171,9 +171,7 @@ static CURLcode gopher_do(struct connectdata *conn, bool *done)
   free(sel_org);
 
   if(!result)
-    /* We can use Curl_sendf to send the terminal \r\n relatively safely and
-       save allocing another string/doing another _write loop. */
-    result = Curl_sendf(sockfd, conn, "\r\n");
+    result = Curl_write(conn, sockfd, "\r\n", 2, &amount);
   if(result) {
     failf(data, "Failed sending Gopher request");
     return result;