websocket: fix message send corruption

websocket: fix message send corruption

- Fix a bug in EAGAIN handling when sending frames that led to a
  corrupted last byte of the frame sent.

- Restore sanity to curl_ws_send() behaviour:

  - Partial writes are reported as OK with the actual number of
    payload bytes sent.

  - CURLE_AGAIN is only returned when none of the payload bytes
    (or for 0-length frames, not all of the frame header bytes)
    could be sent.

  - curl_ws_send() now behaves like a common send() call.

- Change 'ws-data' test client to allow concurrent send/recv
  operations and vary frame sizes and repeat count.

- Add DEBUG env var CURL_WS_CHUNK_EAGAIN to simulate blocking
  after a chunk of an encoded websocket frame has been sent.

- Add tests.

Prior to this change data corruption may occur when sending websocket
messages due to two bugs:

1) 3e64569a (precedes 8.10.0) caused a data corruption bug in the last
   byte of frame of large messages.

2) curl_ws_send had non-traditional send behavior and could return
   CURLE_AGAIN with bytes sent and expect the caller to adjust buffer
   and buflen in a subsequent call. That behavior was not documented.

Reported-by: na-trium-144@users.noreply.github.com
Fixes https://github.com/curl/curl/issues/15865
Fixes https://github.com/curl/curl/issues/15865#issuecomment-2569870144
Closes https://github.com/curl/curl/pull/15901