Add support for other ways of triggering intermediate result outputs

Add support for other ways of triggering intermediate result outputs

Fio has support for using the USR1 signal to dump the current
results while continuing to run. Add a --status-interval parameter
to allow this to be configured to automatically happen every X
seconds.

There's also support for signaling fio through the file system. To
do that, simply touch /tmp/fio-dump-status. When fio sees this file,
it will unlink and dump the current results.

A small tweak is needed for the file approach to work in Windows.

Signed-off-by: Jens Axboe <axboe@kernel.dk>