windows: Better handling of Ctrl-C

This builds upon the changes made in 21950463. Using signal() does not
produce reliable results so SetConsoleCtrlHandler is used, and the handler
is installed only once during initialization instead of removing it in
mp_hal_set_interrupt_char when it is not strictly needed anymore, since
removing it might lead to Ctrl-C events being missed because they are
fired on a seperate thread which might only become alive after the handler
was removed.