Fix output of swapinfo in human readable mode

[csjp]

Summary

• 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

I went back through the revision history and didn't see any reason why this was explicitly not done, but this seems more consistent to me.

Test Plan

Before:

$ swapinfo -h
Device 1K-blocks Used Avail Capacity
/dev/gpt/swapfs 1048576 0B 1.0G 0%
$

After:

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