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>
diff --git a/README b/README
index 3782570..15a0731 100644
--- a/README
+++ b/README
@@ -155,6 +155,7 @@
 	--eta=when		When ETA estimate should be printed
 				May be "always", "never" or "auto"
 	--eta-newline=time	Force a new line for every 'time' period passed
+	--status-interval=t	Force full status dump every 't' period passed
 	--section=name		Only run specified section in job file.
 				Multiple sections can be specified.
 	--alloc-size=kb		Set smalloc pool to this size in kb (def 1024)