Update demos

To build demos, run
`cmake .. -DENABLE_DEMOS=ON`

demos/src/sm2_key_export_demo.c

@@ -0,0 +1,64 @@
+/*
+ *  Copyright 2014-2023 The GmSSL Project. All Rights Reserved.
+ *
+ *  Licensed under the Apache License, Version 2.0 (the License); you may
+ *  not use this file except in compliance with the License.
+ *
+ *  http://www.apache.org/licenses/LICENSE-2.0
+ */
+
+
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+#include <gmssl/mem.h>
+#include <gmssl/sm2.h>
+
+
+int main(int argc, char **argv)
+{
+	int ret = -1;
+	char *prog = argv[0];
+	char *keyfile;
+	char *pass;
+	FILE *keyfp = NULL;
+	SM2_KEY sm2_key;
+
+	if (argc < 3) {
+		fprintf(stderr, "usage: %s <key.pem> <pass>\n", prog);
+		return -1;
+	}
+	keyfile = argv[1];
+	pass = argv[2];
+
+	if (!(keyfp = fopen(keyfile, "rb"))) {
+		fprintf(stderr, "%s: open file '%s' failure\n", prog, keyfile);
+		return -1;
+	}
+	if (sm2_private_key_info_decrypt_from_pem(&sm2_key, pass, keyfp) != 1) {
+		fprintf(stderr, "%s: load key failure\n", prog);
+		goto end;
+	}
+	if (sm2_private_key_info_to_pem(&sm2_key, stdout) != 1) {
+		fprintf(stderr, "%s: export failure\n", prog);
+		goto end;
+	}
+	ret = 0;
+
+end:
+	gmssl_secure_clear(&sm2_key, sizeof(sm2_key));
+	if (keyfp) fclose(keyfp);
+	return ret;
+}
+
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