This function brings the PPP connection down. The
handle argument is the result of a successful
call to cyg_ppp_up(). This function only signals
to the PPP subsystem that the link should be brought down. The link
will be terminated asynchronously. If the application needs to wait
for the link to terminate, then it should call
cyg_ppp_wait_down() after calling
cyg_ppp_down().
The function returns zero if it was able to start the termination of the PPP connection successfully. It will return -1 if the connection is not running, or if it could not otherwise start the termination.