Support limited mixed command line options and job file

With this, you can now do:

$ fio --timeout=10s --time_based=1 jobfile.fio

and have the command line options work as if they were part
of the global section of jobfile.fio

Note that more advanced uses, like:

$ fio --option_foo=1 jobfile.fio --option_bar=1 jobfile2.fio

will not work, we still clear to default options for
subsequent jobs.

Signed-off-by: Jens Axboe <axboe@fb.com>