[PREVIEW] Encapsulate SSL configuration settings

This creates an Ssl::Config class to manage SSL configuration settings
and creates a ConfigOptions sub-class to hold the settings which are
needed passing around in the SSL code.

Functional changes are all in the parsing and SSL options for
cache_peer, http(s)_port are aligned and a new directive
ssl_proxy_direct is added to combine the DIRECT SSL connection settings
with the same set of options.

TODO:
* implement dump/free functions properly
* implement parse for backward-compatible loading of the old DIRECT SSL
settings options.

Amos