Implement a ProxyTLSEngine setting to match frontend client behavior

[Castaglia]

The scope of this ticket is to implement a ProxyTLSEngine setting that tells mod_proxy to "do what the frontend client does". Thus if the frontend client uses FTPS, then mod_proxy would use FTPS to the backend server; this should include whether the it is implicit or explicit FTPS. And if the frontend client does not use FTPS, then mod_proxy won't use FTPS to the backend servers.

Note that such a setting will do what the frontend client is doing, at the time that mod_proxy connects to the backend server (which, in turn, is affected by such things as the ProxyReverseConnectPolicy).

[Castaglia]

This has been implemented in master.