Fix problem with --showcmd and callbacks that verify

Fix problem with --showcmd and callbacks that verify

David reports:

I'm using "fio --showcmd file.job" to convert a script to plain
commandline arguments and this fails if the directory specified in job
file does not exist. While this has to be an error if the job is being
executed, it should not be in context of --showcmd.

To reproduce:
$ cat job.fio
[global]
directory=/x

$ fio --showcmd job.fio
fio: /x is not a directory
fio: failed parsing directory=/x
fio: job global dropped

Expected output:
fio --directory=/x

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