... in most cases instead of 'struct connectdata *' but in some cases in
addition to.

- We mostly operate on transfers and not connections.

- We need the transfer handle to log, store data and more. Everything in
  libcurl is driven by a transfer (the CURL * in the public API).

- This work clarifies and separates the transfers from the connections
  better.

- We should avoid "conn->data". Since individual connections can be used
  by many transfers when multiplexing, making sure that conn->data
  points to the current and correct transfer at all times is difficult
  and has been notoriously error-prone over the years. The goal is to
  ultimately remove the conn->data pointer for this reason.

Closes #6425

Closes #6172

... as the only remaining user of that function. Also fix gopher.c to
instead use Curl_write()

Closes #6020

Closes #3144

The struct field is never set (since 5e0d9aea3) so remove the use of it
and remove the connectdata pointer from the prototype.

Reported-by: Tejas
Bug: https://curl.haxx.se/mail/lib-2018-06/0054.html
Closes #2647

- Get rid of variable that was generating false positive warning
(unitialized)

- Fix issues in tests

- Reduce scope of several variables all over

etc

Closes #2631

Reported-by: Guido Berhoerster
Fixes #2314
Closes #2316

- Check for pending data before waiting on the socket.

Bug: https://github.com/curl/curl/issues/1156
Reported-by: Adam Langley