Additional options added.

Added:
- You can now overwrite the user agent using "--user-agent <agent
string>" option.

- Added support for the "--globoff" cURL option.

- Added support for the "--interface <name>" cURL option.

Fixed:
- Invalid exit code in do_handle_http_status() corrected.

- Because parse_options() did not update the "has_url" flag, the URL
could be overwritten.

- do_handle_http_status() is now more robust. Sometimes, not only the
HTTP status code was written to the status file. This should now be
detected and handled.