ceph-post-file: use getopt for multiple options, add longopts to help

Fixes: #7993
Signed-off-by: Dan Mick <dan.mick@inktank.com>
Merge pull request #1605 from ceph/wip-7993

ceph-post-file: use getopt for multiple options, add longopts to help

Reviewed-by: Sage Weil <sage@inktank.com>