Closes #5325

Reported by the new script 'scripts/copyright.pl'. The script has a
regex whitelist for the files that don't need copyright headers.

Removed three (mostly usesless) README files from docs/

Closes #5141

Fixes build.

Closes #5132

- Implement new option CURLSSLOPT_REVOKE_BEST_EFFORT and
  --ssl-revoke-best-effort to allow a "best effort" revocation check.

A best effort revocation check ignores errors that the revocation check
was unable to take place. The reasoning is described in detail below and
discussed further in the PR.

---

When running e.g. with Fiddler, the schannel backend fails with an
unhelpful error message:

	Unknown error (0x80092012) - The revocation function was unable
	to check revocation for the certificate.

Sadly, many enterprise users who are stuck behind MITM proxies suffer
the very same problem.

This has been discussed in plenty of issues:
https://github.com/curl/curl/issues/3727,
https://github.com/curl/curl/issues/264, for example.

In the latter, a Microsoft Edge developer even made the case that the
common behavior is to ignore issues when a certificate has no recorded
distribution point for revocation lists, or when the server is offline.
This is also known as "best effort" strategy and addresses the Fiddler
issue.

Unfortunately, this strategy was not chosen as the default for schannel
(and is therefore a backend-specific behavior: OpenSSL seems to happily
ignore the offline servers and missing distribution points).

To maintain backward-compatibility, we therefore add a new flag
(`CURLSSLOPT_REVOKE_BEST_EFFORT`) and a new option
(`--ssl-revoke-best-effort`) to select the new behavior.

Due to the many related issues Git for Windows and GitHub Desktop, the
plan is to make this behavior the default in these software packages.

The test 2070 was added to verify this behavior, adapted from 310.

Based-on-work-by: georgeok <giorgos.n.oikonomou@gmail.com>
Co-authored-by: Markus Olsson <j.markus.olsson@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Closes https://github.com/curl/curl/pull/4981

... and check for inconsistencies for OS400 at build time with the new
chkstrings tool.

Closes #4822

- Add new error code CURLE_QUIC_CONNECT_ERROR for QUIC connection
  errors.

Prior to this change CURLE_FAILED_INIT was used, but that was not
correct.

Closes https://github.com/curl/curl/pull/4754

- Add new error CURLM_BAD_FUNCTION_ARGUMENT and return that error when
  curl_multi_wait/poll is passed timeout param < 0.

Prior to this change passing a negative value to curl_multi_wait/poll
such as -1 could cause the function to wait forever.

Reported-by: hamstergene@users.noreply.github.com

Fixes https://github.com/curl/curl/issues/4763

Closes https://github.com/curl/curl/pull/4765

Bug: https://github.com/curl/curl/pull/4754#issuecomment-569126922
Reported-by: Emil Engler

Add support for CURLSSLOPT_NO_PARTIALCHAIN in CURLOPT_PROXY_SSL_OPTIONS
and OS400 package spec.

Also I added the option to the NameValue list in the tool even though it
isn't exposed as a command-line option (...yet?). (NameValue stringizes
the option name for the curl cmd -> libcurl source generator)

Follow-up to 564d88a which added CURLSSLOPT_NO_PARTIALCHAIN.

Ref: https://github.com/curl/curl/pull/4655

This commit adds curl_multi_wakeup() which was previously in the TODO
list under the curl_multi_unblock name.

On some platforms and with some configurations this feature might not be
available or can fail, in these cases a new error code
(CURLM_WAKEUP_FAILURE) is returned from curl_multi_wakeup().

Fixes #4418
Closes #4608