[plt-scheme] Flag change to Help Desk launchers

Note: The -x flag to the Help Desk and Background Help Desk launchers
has been changed to -r.  The -x was a mnemonic for "external
connections"; the new -r is a mnemonic for "remote connections".  

The change was made because the word "external" in the Help Desk code
had come to be associated with browsers-other-than-the-PLT-browser.

Please change your scripts if needed.

-- Paul