Resolve #621: Config option for allowed SSL ciphers

In the global section, for example:

    SSLCiphers = ECDH+AESGCM:DH+AESGCM:ECDH+AES256:DH+AES256:ECDH+AES128:DH+AES:ECDH+3DES:DH+3DES:RSA+AESGCM:RSA+AES:RSA+3DES:!aNULL:!MD5:!DSS

https://www.openssl.org/docs/apps/ciphers.html
https://hynek.me/articles/hardening-your-web-servers-ssl-ciphers
Merge pull request #717 from jpnurmi/ssl-ciphers

Resolve #621: Config option for allowed SSL ciphers