cfilters: send flush

Since data can be held in connection filter buffers when sending gives
EAGAIN, add methods to query this and perform flushing of those buffers.

The transfer loop will continue sending until all upload data is
processed and the connection is flushed.

- add `CF_QUERY_SEND_PENDING` to query filters
- add `CF_CTRL_DATA_SEND_FLUSH` to flush filters
- change `Curl_req_want_send()` to query the connection
  if it needs flushing
- use `Curl_req_want_send()` to determine the POLLOUT
  in the PERFORMING multi state
- implement flush handling in the HTTP/2 connection filter

Closes #14271

lib/cfilters.h

@@ -141,6 +141,7 @@ typedef CURLcode Curl_cft_conn_keep_alive(struct Curl_cfilter *cf,
 /* update conn info at connection and data */
 #define CF_CTRL_CONN_INFO_UPDATE (256+0) /* 0          NULL     ignored */
 #define CF_CTRL_FORGET_SOCKET    (256+1) /* 0          NULL     ignored */
+#define CF_CTRL_FLUSH            (256+2) /* 0          NULL     first fail */
 
 /**
  * Handle event/control for the filter.
@@ -163,6 +164,7 @@ typedef CURLcode Curl_cft_cntrl(struct Curl_cfilter *cf,
  *                   were received.
  *                   -1 if not determined yet.
  * - CF_QUERY_SOCKET: the socket used by the filter chain
+ * - CF_QUERY_NEED_FLUSH: TRUE iff any of the filters have unsent data
  */
 /*      query                             res1       res2     */
 #define CF_QUERY_MAX_CONCURRENT     1  /* number     -        */
@@ -171,6 +173,7 @@ typedef CURLcode Curl_cft_cntrl(struct Curl_cfilter *cf,
 #define CF_QUERY_TIMER_CONNECT      4  /* -          struct curltime */
 #define CF_QUERY_TIMER_APPCONNECT   5  /* -          struct curltime */
 #define CF_QUERY_STREAM_ERROR       6  /* error code - */
+#define CF_QUERY_NEED_FLUSH         7  /* TRUE/FALSE - */
 
 /**
  * Query the cfilter for properties. Filters ignorant of a query will
@@ -401,6 +404,17 @@ CURLcode Curl_conn_shutdown(struct Curl_easy *data, int sockindex, bool *done);
 bool Curl_conn_data_pending(struct Curl_easy *data,
                             int sockindex);
 
+/**
+ * Return TRUE if any of the connection filters at chain `sockindex`
+ * have data still to send.
+ */
+bool Curl_conn_needs_flush(struct Curl_easy *data, int sockindex);
+
+/**
+ * Flush any pending data on the connection filters at chain `sockindex`.
+ */
+CURLcode Curl_conn_flush(struct Curl_easy *data, int sockindex);
+
 /**
  * Return the socket used on data's connection for the index.
  * Returns CURL_SOCKET_BAD if not available.