Make sure each job loads a private io engine

Threads got it shared, breaks for obvious reasons. Also gets rid
of the free hack in ioengine unload.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/fio.h b/fio.h
index 12cf3c9..c20f21b 100644
--- a/fio.h
+++ b/fio.h
@@ -307,6 +307,7 @@
 	char *name;
 	char *directory;
 	char *filename;
+	char *ioengine;
 	char verror[128];
 	pthread_t thread;
 	int thread_number;