websocket: introduce blocking sends

When using `curl_ws_send()`, perform a blocking send of the data under
the following conditions:

- the websocket is in raw mode and the call is done from within a curl
  callback. A partial write of the data could subsequently mess up the
  ws framing, as a callback has a hard time handling this.

- the websocket is encoding the data itself, has added it to its
  internal sendbuf. A partial flush of the buffer has unclear semantics
  for the caller, as they will have no idea what to send again.

Fixes WebSockets tests with CURL_DBG_SOCK_WBLOCK=90 set.
Closes #14458

docs/TODO

@@ -479,6 +479,8 @@
 
  - curl_multi_remove_handle for any of the above. See section 2.3.
 
+ - Calling curl_ws_send() from a callback
+
 2.2 Better support for same name resolves
 
  If a name resolve has been initiated for name NN and a second easy handle

docs/libcurl/curl_global_trace.md

@@ -109,6 +109,10 @@ Traces reading of upload data from the application in order to send it to the se
 
 Traces writing of download data, received from the server, to the application.
 
+## `ws`
+
+Tracing of WebSocket operations when this protocol is enabled in your build.
+
 # TRACE GROUPS
 
 Besides the specific component names there are the following group names

docs/libcurl/curl_ws_send.md

@@ -53,6 +53,10 @@ If **CURLWS_RAW_MODE** is enabled in CURLOPT_WS_OPTIONS(3), the
 To send a message consisting of multiple frames, set the *CURLWS_CONT* bit
 in all frames except the final one.
 
+Warning: while it is possible to invoke this function from a callback,
+such a call is blocking in this situation, e.g. only returns after all data
+has been sent or an error is encountered.
+
 # FLAGS
 
 ## CURLWS_TEXT