main: set $COLUMNS/$ROWS for PID 1 based on /dev/console data

main: set $COLUMNS/$ROWS for PID 1 based on /dev/console data

In PID 1 we write status information to /dev/console regularly, but we
cannot keep it open continously, due to the kernel's SAK logic (which
would kill PID 1 if user hits SAK). But closing/reopening it all the
time really sucks for tty types that have no window size management
(such as serial terminals/hvc0 and suchlike), because it also means the
TTY is fully closed most of the time, and that resets the window sizes
to 0/0.

Now, we reinitialize the window size on every reopen, but that is a bit
expensive for simple status output. Hence, cache the window size in the
usualy $COLUMNS/$ROWS environment variables. We don't inherit these to
our payloads anyway, hence these are free to us to use.