Display short usage when no arguments given

This patch adds a usage message to the help for `argc<2`:

    Usage: curl [options...] <url>
      Transfers data at <url> to standard output.
    curl: try 'curl --help' or 'curl --manual' for more information

The second line makes it clear that curl != wget, so novice users get a hint that they can expect a lot of garbage on their terminals :)