init: allow FIO_OPT_STR_SET options to have an optional argument

Makes them work like a bool.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
init: escape description option string

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Merge branch 'master' of ssh://git.kernel.dk/data/git/fio