schannel: add "best effort" revocation check option

- 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

docs/cmdline-opts/Makefile.inc

@@ -179,6 +179,7 @@ DPAGES =					\
   ssl-allow-beast.d				\
   ssl-no-revoke.d				\
   ssl-reqd.d					\
+  ssl-revoke-best-effort.d			\
   ssl.d						\
   sslv2.d sslv3.d				\
   stderr.d					\

docs/cmdline-opts/ssl-revoke-best-effort.d

@@ -0,0 +1,7 @@
+Long: ssl-revoke-best-effort
+Help: Ignore missing/offline cert CRL distribution points (Schannel)
+Added: 7.70.0
+---
+(Schannel) This option tells curl to ignore certificate revocation checks when
+they failed due to missing/offline distribution points for the revocation check
+lists.

docs/libcurl/opts/CURLOPT_PROXY_SSL_OPTIONS.3

@@ -49,6 +49,13 @@ Tells libcurl to not accept "partial" certificate chains, which it otherwise
 does by default. This option is only supported for OpenSSL and will fail the
 certificate verification if the chain ends with an intermediate certificate
 and not with a root cert. (Added in 7.68.0)
+
+.IP CURLSSLOPT_REVOKE_BEST_EFFORT
+Tells libcurl to ignore certificate revocation checks in case of missing or
+offline distribution points for those SSL backends where such behavior is
+present. This option is only supported for Schannel (the native Windows SSL
+library). If combined with \fICURLSSLOPT_NO_REVOKE\fP, the latter takes
+precedence. (Added in 7.70.0)
 .SH DEFAULT
 0
 .SH PROTOCOLS

docs/libcurl/opts/CURLOPT_SSL_OPTIONS.3

@@ -49,6 +49,12 @@ Tells libcurl to not accept "partial" certificate chains, which it otherwise
 does by default. This option is only supported for OpenSSL and will fail the
 certificate verification if the chain ends with an intermediate certificate
 and not with a root cert. (Added in 7.68.0)
+.IP CURLSSLOPT_REVOKE_BEST_EFFORT
+Tells libcurl to ignore certificate revocation checks in case of missing or
+offline distribution points for those SSL backends where such behavior is
+present. This option is only supported for Schannel (the native Windows SSL
+library). If combined with \fICURLSSLOPT_NO_REVOKE\fP, the latter takes
+precedence. (Added in 7.70.0)
 .SH DEFAULT
 0
 .SH PROTOCOLS

docs/libcurl/symbols-in-versions

@@ -746,6 +746,7 @@ CURLSSLOPT_NO_PARTIALCHAIN      7.68.0
 CURLSSLOPT_NO_REVOKE            7.44.0
 CURLSSLSET_NO_BACKENDS          7.56.0
 CURLSSLSET_OK                   7.56.0
+CURLSSLOPT_REVOKE_BEST_EFFORT   7.70.0
 CURLSSLSET_TOO_LATE             7.56.0
 CURLSSLSET_UNKNOWN_BACKEND      7.56.0
 CURLUE_BAD_HANDLE               7.62.0