websocket: introduce blocking sends

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