Btrfs-progs: enhance btrfs qgroup to print the result as a table

This patch enhance to print the result as a table.

You can use it like:
	btrfs qgroup show <path>
However, to table the result better, we make '-p' and '-c' not present
at the same time.

For example:

	btrfs qgroup show -pr <path>
The result will output as the follow format:

qgroupid rfer       excl       max_excl       parent
-------- ----       ----       --------       ------
0/265    1289752576 1289752576 0              ---
1/0      0          0          10999511627776 2/0,3/0
2/0      0          0          0              ---
3/0      0          0          0              ---

Signed-off-by: Wang Shilong <wangsl-fnst@cn.fujitsu.com>
Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Btrfs-progs: add '--block-size' option to control print result

You can use it like:
	btrfs qgroup show --block-size=m <mnt>

Here, block size supports k/K/m/M/g/G/t/T/p/P/e/E.

Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Btrfs-progs: make pretty_size_snprintf() return len

Sometimes, we need to catch length of snprintf() in pretty_size_snprintf().

Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Btrfs-progs: add '-h' options to print sizes in human readable format

This is a short cut for people who don't want to use '--block-size',it
will print sizes in human readable format.

Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>