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/eta.c b/eta.c
index b367ce9..e1cced4 100644
--- a/eta.c
+++ b/eta.c
@@ -7,7 +7,7 @@
 
 #include "fio.h"
 
-static char run_str[MAX_JOBS + 1];
+static char run_str[REAL_MAX_JOBS + 1];
 
 /*
  * Sets the status of the 'td' in the printed status map.