Add --max-jobs/-j command line option

And shrink OSX max default number of jobs supported. OSX has,
by default, a very small shared memory segment available.

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
diff --git a/os/os.h b/os/os.h
index 1569e40..4bbdd62 100644
--- a/os/os.h
+++ b/os/os.h
@@ -106,6 +106,10 @@
 #define FIO_PREFERRED_ENGINE	"sync"
 #endif
 
+#ifndef FIO_MAX_JOBS
+#define FIO_MAX_JOBS		2048
+#endif
+
 #ifndef FIO_HAVE_BLKTRACE
 static inline int is_blktrace(const char *fname)
 {