Updated test 1560 to verify.

Closes #3880

If --with-ssl is used and configure still couldn't enable SSL this
creates an error instead of just silently ignoring the fact.

Suggested-by: Isaiah Norton
Fixes #3824
Closes #3830

No need to set variables to zero as calloc() does this for us.

Closes #3879

Clues-provided-by: Jay Satiro
Clues-provided-by: Jeroen Ooms
Fixes #3711
Closes #3874

In Curl_multissl_version() it was possible to overflow the passed in
buffer if the generated version string exceeded the size of the buffer.
Fix by inverting the logic, and also make sure to not exceed the local
buffer during the string generation.

Closes #3863
Reported-by: nevv on HackerOne/curl
Reviewed-by: Jay Satiro
Reviewed-by: Daniel Stenberg

For consistency and to a avoid confusion.

Closes #3869

Reported-by: Joombalaya on github
Fixes #3818

I missed this one in commit 6b3dde7f.

Codacy/CppCheck warns about this. Consistently use parentheses as we
already do in some places to silence the warning.

Closes https://github.com/curl/curl/pull/3866

I broke it in d1b5cf83 and
97de97da.

Reported-by: Viktor Szakats
Ref: https://github.com/curl/curl/commit/97de97daefc2ed084c91eff34af2426f2e55e134#commitcomment-33499044
Closes https://github.com/curl/curl/pull/3868

Missed in f0bdd72c.

Closes #3867

Closes #3861

I missed these in commit d1b5cf83.

Due to limitations in Curl_resolver_wait_resolv(), it doesn't work for
DOH resolves. This fix disables DOH for those.

Limitation added to KNOWN_BUGS.

Fixes #3850
Closes #3857

.. because we allow snprintf use in docs/examples.

Closes https://github.com/curl/curl/pull/3862

...and misalignment of these comments. From a78c61a4.

Closes #3860

This reverts commit b0972bc8.

- No longer show verbose output for the conncache closure handle.

The offending commit was added so that the conncache closure handle
would inherit verbose mode from the user's easy handle. (Note there is
no way for the user to set options for the closure handle which is why
that was necessary.) Other debug settings such as the debug function
were not also inherited since we determined that could lead to crashes
if the user's per-handle private data was used on an unexpected handle.

The reporter here says he has a debug function to capture the verbose
output, and does not expect or want any output to stderr; however
because the conncache closure handle does not inherit the debug function
the verbose output for that handle does go to stderr.

There are other plausible scenarios as well such as the user redirects
stderr on their handle, which is also not inherited since it could lead
to crashes when used on an unexpecte...

From 6012fa5a.

Closes #3858

Closes #3839

Fixes #3726
Closes #3849

Fixes #3726
Closes #3849

There is need to use @ on every command once echo has been turned off.

Closes #3854

- Do not switch to HTTP/2 for an HTTP proxy that is not tunnelling to
  the destination host.

We already do something similar for HTTPS proxies by not sending h2. [1]

Prior to this change setting CURL_HTTP_VERSION_2_PRIOR_KNOWLEDGE would
incorrectly use HTTP/2 to talk to the proxy, which is not something we
support (yet?). Also it's debatable whether or not that setting should
apply to HTTP/2 proxies.

[1]: https://github.com/curl/curl/commit/17c5d05

Bug: https://github.com/curl/curl/issues/3570
Bug: https://github.com/curl/curl/issues/3832

Closes https://github.com/curl/curl/pull/3853

Closes https://github.com/curl/curl/pull/3842