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/fio.h b/fio.h
index 50a868a..ebf6309 100644
--- a/fio.h
+++ b/fio.h
@@ -362,6 +362,7 @@
 extern int is_backend;
 extern int nr_clients;
 extern int log_syslog;
+extern int status_interval;
 extern const char fio_version_string[];
 
 extern struct thread_data *threads;