support OPENSSL to cmake script

Signed-off-by: Jack <jack.wgm@gmail.com>
1. 修复avhttp在处理https连接握手时的错误.
2. 在multi_download的实现中, 默认为不检查证书认证.
3. 修复在异步读取数据时远程断开, error_code为eof, 数据不被保存.

Signed-off-by: Jack <jack.wgm@gmail.com>