MEDIUM: ssl: add support for ciphersuites option for TLSv1.3

OpenSSL released support for TLSv1.3. It also added a separate function
SSL_CTX_set_ciphersuites that is used to set the ciphers used in the
TLS 1.3 handshake. This change adds support for that new configuration
option by adding a ciphersuites configuration variable that works
essentially the same as the existing ciphers setting.

Note that it should likely be backported to 1.8 in order to ease usage
of the now released openssl-1.1.1.
diff --git a/include/types/server.h b/include/types/server.h
index 8adc29b..a2e11c3 100644
--- a/include/types/server.h
+++ b/include/types/server.h
@@ -288,6 +288,9 @@
 			int allocated_size;
 		} * reused_sess;
 		char *ciphers;			/* cipher suite to use if non-null */
+#if (OPENSSL_VERSION_NUMBER >= 0x10101000L && !defined OPENSSL_IS_BORINGSSL && !defined LIBRESSL_VERSION_NUMBER)
+		char *ciphersuites;			/* TLS 1.3 cipher suite to use if non-null */
+#endif
 		int options;			/* ssl options */
 		int verify;			/* verify method (set of SSL_VERIFY_* flags) */
 		struct tls_version_filter methods;	/* ssl methods */