Closes #5892

When this error code was introduced in b6a53fff6c1d07e8a9, it was
forgotten to be added in the errors array and doh_strerror function.

Closes #5863

Closes https://github.com/curl/curl/pull/5704

To reduce the amount of allocations needed for creating a Curl_addrinfo
struct, make a single larger malloc instead of three separate smaller
ones.

Closes #5533

When USE_RESOLVE_ON_IPS is set (defined on macOS), it means that
numerical IP addresses still need to get "resolved" - but not with DoH.

Reported-by: Viktor Szakats
Fixes #5454
Closes #5459

 - Stick to a single unified way to use structs
 - Make checksrc complain on 'typedef struct {'
 - Allow them in tests, public headers and examples

 - Let MD4_CTX, MD5_CTX, and SHA256_CTX typedefs remain as they actually
   typedef different types/structs depending on build conditions.

Closes #5338

A common set of functions instead of many separate implementations for
creating buffers that can grow when appending data to them. Existing
functionality has been ported over.

In my early basic testing, the total number of allocations seem at
roughly the same amount as before, possibly a few less.

See docs/DYNBUF.md for a description of the API.

Closes #5300

Closes #5306

- 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

Closes #4869
Reviewed-by: Emil Engler and Daniel Gustafsson