ceph-filestore-dump.cc: don't use po::value<string>()->required()

Don't use po::value<string>()->required() since this breaks build on
RHEL/CentOs6. Check if the options are set as in the code of other
ceph parts.

Move some checks up in the code to validate options as soon
as possible. Remove printing 'help' twice, and check it first.

Fix type description.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
Signed-off-by: David Zafman <david.zafman@inktank.com>