btrfs-progs: add options to tune units for fi df output

The size unit format is a longstanding annoyance. This patch is based on
the work of Nils and Alexandre and enhances the options. It's possible
to select raw bytes, SI-based or IEC-based compact units (human
frientdly) or a fixed base from kilobytes to terabytes. The default is
compact human readable IEC-based, no change to current version.

CC: Nils Steinger <nst@voidptr.de>
CC: Alexandre Oliva <oliva@gnu.org>
Signed-off-by: David Sterba <dsterba@suse.cz>