CURLOPT_DEFAULT_PROTOCOL: added

- Add new option CURLOPT_DEFAULT_PROTOCOL to allow specifying a default
protocol for schemeless URLs.

- Add new tool option --proto-default to expose
CURLOPT_DEFAULT_PROTOCOL.

In the case of schemeless URLs libcurl will behave in this way:

When the option is used libcurl will use the supplied default.

When the option is not used, libcurl will follow its usual plan of
guessing from the hostname and falling back to 'http'.