Fix segfault is bool option is used on command line and fails parsing

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Better catch arguments with required options and none passed

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Make group_reporting be a string-set option again

Otherwise it'll break job files between 2.1 and 2.0.x and earlier.

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