tools/rados: Allow binary file output of omap data

Extends getomapval and getompaheader to take an output
file argument the same way 'get' does.  Removes misleading
'-o' line from usage(), there doesn't appear to be any
such
option.

Signed-off-by: John Spray <john.spray@inktank.com>
Merge pull request #1479 from ceph/wip-rados-outfile

tools/rados: Allow binary file output of omap data

Reviewed-by: Sage Weil <sage@inktank.com>