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/fio.c b/fio.c
index 5ea1a2c..01aa1d6 100644
--- a/fio.c
+++ b/fio.c
@@ -1515,7 +1515,7 @@
 	set_genesis_time();
 
 	while (todo) {
-		struct thread_data *map[MAX_JOBS];
+		struct thread_data *map[REAL_MAX_JOBS];
 		struct timeval this_start;
 		int this_jobs = 0, left;