Allow bookmark short name in 'zfs bookmark'

This lets users create a bookmark from the command line by its name
only, without the need to specify the dataset path which is extacted
from the snapshot parameter.

These commands are now equivalent:

   zfs bookmark poolname/fs@snap poolname/fs#bookmark
   zfs bookmark @snap poolname/fs#bookmark
   zfs bookmark poolname/fs@snap \#bookmark

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #3665
Closes #6027