When a ssh session startup fails, it is useful to know why it has
failed. This commit changes the message from:
   "Failure establishing ssh session"
to something like this, for example:
   "Failure establishing ssh session: -5, Unable to exchange encryption keys"

Closes #3481

Follow-up to 2fa0d57e. The __xlc__ symbol is only defined there if a
particular flag is used for legacy macros.

Fixes #3474
Closes #3479

.... to not pass in a const in the second argument as that's not how it
is supposed to be used and might cause compiler warnings.

Reported-by: Pavel Pavlov
Fixes #3477
Closes #3478

Since it isn't totally clang compatible, we detect this IBM clang
front-end and if detected, avoids some clang specific magic.

Reported-by: Kees Dekker
Fixes #3474
Closes #3476

[skip ci]

extract_if_dead() dead is called from two functions, and only one of
them should get conn->data updated and now neither call path clears it.

scan-build found a case where conn->data would be NULL dereferenced in
ConnectionExists() otherwise.

Closes #3473

Found by scan-build. Follow-up to 4c35574b.

Closes #3471

Since they're used purely for testing purposes, I think they should
rather be stored there.

Closes #3470

Closes #3469

zlibwapi.lib (dynamic library) and zlibstat.lib (static library) have
the ZLIB_WINAPI define set by default. Using them requires that define
too.

Ref: https://zlib.net/DLL_FAQ.txt

Fixes https://github.com/curl/curl/issues/3133
Closes https://github.com/curl/curl/pull/3460

Make sure that this function sets a proper "live" transfer for the
connection before calling the protocol-specific connection check
function, and then clear it again afterward as a non-used connection has
no current transfer.

Reported-by: Jeroen Ooms
Reviewed-by: Marcel Raad
Reviewed-by: Daniel Gustafsson
Fixes #3463
Closes #3464

OpenSSL_version() replaces OpenSSL_version_num()

Closes #3462

We use "conn" everywhere to be a pointer to the connection.

Introduces two functions that "attaches" and "detaches" the connection
to and from the transfer.

Going forward, we should favour using "data->conn" (since a transfer
always only has a single connection or none at all) to "conn->data"
(since a connection can have none, one or many transfers associated with
it and updating conn->data to be correct is error prone and a frequent
reason for internal issues).

Closes #3442

When calculating the progress bar width.

Reported-by: Peng Li
Fixes #3456
Closes #3458

Invoking the maintainer intended COPYRIGHTYEAR check for everyone
in the PR pipeline is too invasive, especially at the turn of the
year when many files get affected. Remove and leave it as a tool
for maintainers to verify patches before commits.

This reverts f7bdf4b2.

After discussion with: Daniel Stenberg

Closes #3125

Closes #3229

See https://github.com/curl/curl/issues/3453#issuecomment-453054458

Reported-by: jonrumsey on github

Reported-by: jonrumsey on github
Closes #3453

Attempt to reproduce issue #3444.

Closes #3447

Both user and password are cleared uncondtitionally.

Added unit test 1621 to verify.

Fixes #3423
Closes #3433

Added test 1562 to verify.

Reported-by: Jeroen Ooms
Fixes #3445
Closes #3450

Fixes #3436
Closes #3448

 Problem 1

After LOTS of scratching my head, I eventually realized that even when doing
10 uploads in parallel, sometimes the socket callback to the application that
tells it what to wait for on the socket, looked like it would reflect the
status of just the single transfer that just changed state.

Digging into the code revealed that this was indeed the truth. When multiple
transfers are using the same connection, the application did not correctly get
the *combined* flags for all transfers which then could make it switch to READ
(only) when in fact most transfers wanted to get told when the socket was
WRITEABLE.

 Problem 1b

A separate but related regression had also been introduced by me when I
cleared connection/transfer association better a while ago, as now the logic
couldn't find the connection and see if that was marked as used by more
transfers and then it would also prematurely remove the socket from the socket
hash table even in times other transfers were still using it!

 Fix 1

Make sure that each socket stored in the socket hash has a "combined" action
field of what to ask the application to wait for, that is potentially the ORed
action of multiple parallel transfers. And remove that socket hash entry only
if there are no transfers left using it.

 Problem 2

The socket hash entry stored an association to a single transfer using that
socket - and when curl_multi_socket_action() was called to tell libcurl about
activities on that specific socket only that transfer was "handled".

This was WRONG, as a single socket/connection can be used by numerous parallel
transfers and not necessarily a single one.

 Fix 2

We now store a list of handles in the socket hashtable entry and when libcurl
is told there's traffic for a particular socket, it now iterates over all
known transfers using that single socket.

[skip ci]

Closes #3417

[skip ci]

Bug: https://curl.haxx.se/mail/lib-2018-12/0088.html

Closes #3449

similar to commit f508d29f

Closes #3443

Added Curl_resolver_kill() for all three resolver modes, which only
blocks when necessary, along with test 1592 to confirm
curl_multi_remove_handle() doesn't block unless it must.

Closes #3428
Fixes #3371

This reverts commit 07ebaf83.

This also reopens PR #3275 which brought the change now reverted.

Fixes #3384
Closes #3439

This allows programs to include curl/urlapi.h directly.

Reviewed-by: Daniel Gustafsson
Reported-by: Ben Kohler
Fixes #3438
Closes #3441

Starting with Visual Studio 2017 Update 9, Visual Studio doesn't like
the MinimalRebuild option anymore and warns:

cl : Command line warning D9035: option 'Gm' has been deprecated and
will be removed in a future release

The option can be safely removed so that the default is used.

Closes https://github.com/curl/curl/pull/3425