proxy/handler: rework args handling a little bit

[jkarneges]

A few small changes to the args handling:

• Revert config file discovery to the old behavior of explicit arg or single default location. Only the runner should use the more advanced get_config_file() which checks extra paths.
• Revert to creating Settings objects only after checking the config file.
• Remove config file checking from the CliArgs::verify() methods since ProxyApp and HandlerApp already do this. Notably, this avoids having a std::process::exit call in those methods.
• When constructing the array of C strings, use Box for memory management rather than malloc/free.