transfer: conn close on paused upload

transfer: conn close on paused upload

- add 2 variations on test_07_42 which PAUSEs uploads
  and response connections terminating either right away
  or after the 100-continue response
- when detecting the connection being closed in transfer.c
  readwrite_data(), clear ALL send bits in data->req.keepon.
  It no longer makes send to wait for a KEEP_SEND_PAUSE or HOLD.
- in the protocol client writer add the check for incomplete
  response bodies. When an EOS is seen and the length is known,
  check that and fail if bytes are missing.

Reported-by: Sergey Bronnikov
Fixes #13740
Closes #13750