Add |SSL_set_retain_only_sha256_of_client_certs|.

Previously the option to retain only the SHA-256 hash of client
certificates could only be set at the |SSL_CTX| level. This change makes
|SSL| objects inherit the setting from the |SSL_CTX|, but allows it to
be overridden on a per-|SSL| basis.

Change-Id: Id435934af3d425d5f008d2f3b9751d1d0884ee55
Reviewed-on: https://boringssl-review.googlesource.com/12182
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

ssl/test/test_config.cc

diff --git a/ssl/test/test_config.cc b/ssl/test/test_config.cc
index 4f40df9..940e676 100644
--- a/ssl/test/test_config.cc
+++ b/ssl/test/test_config.cc
@@ -108,6 +108,14 @@
   { "-peek-then-read", &TestConfig::peek_then_read },
   { "-enable-grease", &TestConfig::enable_grease },
   { "-use-exporter-between-reads", &TestConfig::use_exporter_between_reads },
+  { "-retain-only-sha256-client-cert-initial",
+    &TestConfig::retain_only_sha256_client_cert_initial },
+  { "-retain-only-sha256-client-cert-resume",
+    &TestConfig::retain_only_sha256_client_cert_resume },
+  { "-expect-sha256-client-cert-initial",
+    &TestConfig::expect_sha256_client_cert_initial },
+  { "-expect-sha256-client-cert-resume",
+    &TestConfig::expect_sha256_client_cert_resume },
 };
 
 const Flag<std::string> kStringFlags[] = {