Fix problem with zdb -d

zdb -d <pool>/<objset ID> does not work when
other command line arguments are included i.e.
zdb -U <cachefile> -d <pool>/<objset ID>
This change fixes the command line parsing
to handle this situation. Also fix issue
where zdb -r <dataset> <file> does not handle
the root <dataset> of the pool. Introduce -N
option to force <objset ID> to be interpreted
as a numeric objsetID.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Rich Ercolani <rincebrain@gmail.com>
Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Signed-off-by: Paul Zuchowski <pzuchowski@datto.com>
Closes #12845
Closes #12944