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>