Fixes a 100% CPU usage on Konsole based terminal

[PartialVolume]

when Konsole terminal exited while nwipe is sitting at the
drive selection screen.

To avoid 100% CPU usage, check for a runaway condition caused by the
function "keystroke = getch() that immediately returns an error
condition. We check for an error condition because getch() returns a
ERR value when the timeout value "timeout( 250 );" expires as well as
when a real error occurs. We can't differentiate from normal operation
and a failure of the getch function to block for the specified period
of timeout. So here we check the while loop hasn't exceeded the number
of expected iterations per second ie. a timeout(250) block value of
250ms means we should not see any more than (1000/250) = 4 iterations.
We double this to 8 to allow a little tolerance. Why is this necessary?
It's been found that in KDE konsole and other terminals based on the QT
terminal engine exiting the terminal without first exiting nwipe
results in nwipe remaining running but detached from any interface
which causes getch to fail and its associated timeout. So the CPU or
CPU core rises to 100%. Here we detect that failure and exit nwipe
gracefully with the appropriate error. This does not affect use of
tmux for attaching or detaching from a running nwipe session when
sitting at the selection screen. All other terminals correctly
terminate nwipe when the terminal itself is exited.