connection: shutdown TLS (for FTP) better

This adds connection shutdown infrastructure and first use for FTP. FTP
data connections, when not encountering an error, are now shut down in a
blocking way with a 2sec timeout.

    - add cfilter `Curl_cft_shutdown` callback
    - keep a shutdown start timestamp and timeout at connectdata
    - provide shutdown timeout default and member in
      `data->set.shutdowntimeout`.
    - provide methods for starting, interrogating and clearing
      shutdown timers
    - provide `Curl_conn_shutdown_blocking()` to shutdown the
      `sockindex` filter chain in a blocking way. Use that in FTP.
    - add `Curl_conn_cf_poll()` to wait for socket events during
      shutdown of a connection filter chain.
      This gets the monitoring sockets and events via the filters
      "adjust_pollset()" methods. This gives correct behaviour when
      shutting down a TLS connection through a HTTP/2 proxy.
    - Implement shutdown for all socket filters
      - for HTTP/2 and h2 proxying to send GOAWAY
      - for TLS backends to the best of their capabilities
      - for tcp socket filter to make a final, nonblocking
        receive to avoid unwanted RST states
    - add shutdown forwarding to happy eyeballers and
      https connect ballers when applicable.

Closes #13904

lib/http_proxy.c

@@ -298,6 +298,7 @@ struct Curl_cftype Curl_cft_http_proxy = {
   http_proxy_cf_destroy,
   http_proxy_cf_connect,
   http_proxy_cf_close,
+  Curl_cf_def_shutdown,
   Curl_cf_http_proxy_get_host,
   Curl_cf_def_adjust_pollset,
   Curl_cf_def_data_pending,