Allow enabling all TLS 1.3 variants by setting |tls13_default|.

Update-Note: Enabling TLS 1.3 now enables both draft-23 and draft-28
by default, in preparation for cycling all to draft-28.
Change-Id: I9405f39081f2e5f7049aaae8a9c85399f21df047
Reviewed-on: https://boringssl-review.googlesource.com/28304
Commit-Queue: Steven Valdez <svaldez@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
Reviewed-by: David Benjamin <davidben@google.com>

tool/server.cc

diff --git a/tool/server.cc b/tool/server.cc
index 7a4e53b..bc90630 100644
--- a/tool/server.cc
+++ b/tool/server.cc
@@ -68,7 +68,8 @@
         "-early-data", kBooleanArgument, "Allow early data",
     },
     {
-        "-tls13-variant", kBooleanArgument, "Enables all TLS 1.3 variants",
+        "-tls13-variant", kOptionalArgument,
+        "Enable the specified experimental TLS 1.3 variant",
     },
     {
         "-www", kBooleanArgument,
@@ -147,6 +148,18 @@
   return x509;
 }
 
+static bool GetTLS13Variant(tls13_variant_t *out, const std::string &in) {
+  if (in == "draft23") {
+    *out = tls13_draft23;
+    return true;
+  }
+  if (in == "draft28") {
+    *out = tls13_draft28;
+    return true;
+  }
+  return false;
+}
+
 static void InfoCallback(const SSL *ssl, int type, int value) {
   switch (type) {
     case SSL_CB_HANDSHAKE_START:
@@ -307,7 +320,13 @@
   }
 
   if (args_map.count("-tls13-variant") != 0) {
-    SSL_CTX_set_tls13_variant(ctx.get(), tls13_draft28);
+    tls13_variant_t variant;
+    if (!GetTLS13Variant(&variant, args_map["-tls13-variant"])) {
+      fprintf(stderr, "Unknown TLS 1.3 variant: %s\n",
+              args_map["-tls13-variant"].c_str());
+      return false;
+    }
+    SSL_CTX_set_tls13_variant(ctx.get(), variant);
   }
 
   if (args_map.count("-debug") != 0) {