- Implement -h (human readable) for the size of the underlying block disk.

- Implement -h (human readable) for the size of the underlying block disk.
  Currently, the size of the swap device is unconditionally reported using
  blocks, even if -h has been used.
- While here, switch to CONVERT_BLOCKS() instead of CONVERT() which will
  avoid overflowing size counters (in human readable form see: r196244)
- Update the column headers to reflect that a size is being reported instead
  of the block size units being used

Before:

$ swapinfo
Device          1K-blocks     Used    Avail Capacity
/dev/gpt/swapfs   1048576        0  1048576     0%
$

After:

$ swapinfo -h
Device           Size     Used    Avail Capacity
/dev/gpt/swapfs    1.0G       0B     1.0G     0%
$

Differential Revision:	https://reviews.freebsd.org/D23758
Reviewed by:	kevans
MFC after:	3 weeks