Add thread_options_pack structure

Add thread_options_pack structure

First step towards having an on-disk and on-wire independent
options structure, with the goal being that we parse options
locally (hence allowing them to be edited sanely) and send them
to the server in one nice package.

Additionally, this is a step towards being able to have a new
fio/gfio "project" file format, which includes all the info a
given client needs. This includes job options, where to connect,
etc.

Signed-off-by: Jens Axboe <axboe@kernel.dk>