Support TLS in Nginx

[eecheng87]

The current implementation doesn't consider TLS processing. However, SSL/TLS have been employed (typically in the form
of HTTPS) by 64.3% of the 137,502 most popular websites on the Internet as reported in November 2018.

Expected modifications:

1. rewrite Makefile, Nginx should be configured with SSL module (--with-http_ssl_module).
2. Since user-land encryption, sendfile is replaced with write. Thus, wrapper of write should be added in ngx.c.
3. Add script to generate RSA/ECDSA key for TLS server.

Similar changes can be referenced in SAIO in the coming weeks.