XPisPort()
  Check if Communication port is available
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   Function:   XPisPort()

               this function checks if the COM port is present, then,
               assuming that a Modem (and a phone) are hooked to it, you
               could use it for dialing for Voice.

   Syntax:     XPisPort([<nPort>]) --> lPortPresent

   Arguments:  <nPort> in the range 0..3 indicating COM1 Though COM4.
               When none supplied, XPisport() looks for COM1.

   Returns:    .t. if a COM port is attached, .f. when there nothing.
               You can only use XPatModem() and XPdtr() when XPisPort()
               returns .t.