Add zfs_nicebytes() to print human-readable sizes

• Add zfs_nicebytes() to print human-readable sizes

Some 'zfs', 'zpool' and 'zdb' output strings can be confusing to the
user when no units are specified. This add a new zfs_nicenum_format
"ZFS_NICENUM_BYTES" used to print bytes in their human-readable form.

Additionally, update some test cases to use machine-parsable 'zfs get'.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #2414
Closes #3185
Closes #3594
Closes #6032