Fix duplicate data in cacheinfo file

Since the cache file is opened for append, truncate it before calling
iniWriteFile().  This isn't create because if it fails, it will
leave the file empty, which will cause the next time this function
is called to do a full request, but at least if fails safe.

src/syncterm/webget.c

@@ -759,7 +759,16 @@ write_cacheinfo(struct http_session *sess)
 		if (!iniSetBool(&info, NULL, "Immutable", sess->cache.immutable, NULL))
 			goto error_return;
 	}
-	bool ret = iniWriteFile(sess->cache_info, info);
+	fflush(sess->cache_info);
+	bool ret;
+	if (chsize(fileno(sess->cache_info), 0) != 0) {
+		fseek(sess->cache_info, 0, SEEK_SET);
+		ret = false;
+	}
+	else {
+		fseek(sess->cache_info, 0, SEEK_SET);
+		ret = iniWriteFile(sess->cache_info, info);
+	}
 	fclose(sess->cache_info);
 	sess->cache_info = NULL;
 	strListFree(&info);