Add "cert" option for rpc server's self-signed SSL certificate

When minerd is used across the internet, the server's rpc userpass
travel unexcrypted, allowing anyone en route to copy them and
empty the server's wallet. Using https: would prevent this
but minerd's SSL connection fails on self-signed certificates.

This change adds a "-cert file" option (cert:"file" in minerd.conf)
to tell minerd the filename of the server's self-signed certificate.