Cedar: Add ProtoOptionString() in PROTO_IMPL, to generate default option values

The WireGuard implementation will have two options that should not have a fixed default value, because they represent two keys (one is preshared, the other is private).

Instead of handling these two options differently in ProtoNewContainer(), this commit adds a new function to PROTO_IMPL: ProtoOptionString().

ProtoOptionString() takes the option's name as argument and returns a heap-allocated string that will be used as value. The function returns NULL when the option doesn't need a randomized value.

src/Cedar/Proto.c

@@ -291,7 +291,7 @@ PROTO_CONTAINER *ProtoContainerNew(const PROTO_IMPL *impl)
 			option->Bool = impl_option->Bool;
 			break;
 		case PROTO_OPTION_STRING:
-			option->String = CopyStr(impl_option->String);
+			option->String = impl_option->String != NULL ? CopyStr(impl_option->String) : impl->OptionStringValue(option->Name);
 			break;
 		default:
 			Debug("ProtoContainerNew(): unhandled option type %u!\n", impl_option->Type);

src/Cedar/Proto.h

@@ -46,6 +46,7 @@ typedef struct PROTO_IMPL
 {
 	const char *(*Name)();
 	const PROTO_OPTION *(*Options)();
+	char *(*OptionStringValue)(const char *name);
 	bool (*Init)(void **param, const LIST *options, CEDAR *cedar, INTERRUPT_MANAGER *im, SOCK_EVENT *se, const char *cipher, const char *hostname);
 	void (*Free)(void *param);
 	bool (*IsPacketForMe)(const PROTO_MODE mode, const void *data, const UINT size);

src/Cedar/Proto_OpenVPN.c

@@ -20,6 +20,7 @@ const PROTO_IMPL *OvsGetProtoImpl()
 	{
 		OvsName,
 		OvsOptions,
+		NULL,
 		OvsInit,
 		OvsFree,
 		OvsIsPacketForMe,

src/Cedar/Proto_SSTP.c

@@ -13,6 +13,7 @@ const PROTO_IMPL *SstpGetProtoImpl()
 	{
 		SstpName,
 		SstpOptions,
+		NULL,
 		SstpInit,
 		SstpFree,
 		NULL,