crash: add the "df" command support

This patch is trying to add "df" support, and its syntax is as below:

[/]% df
            Size            Used           Avail           Used%

        10257271          203796        10053475              2%

[/]% df -k
            Size            Used           Avail           Used%

        10257271          203796        10053475              2%

[/]% df -m
            Size            Used           Avail           Used%

           10017             199            9818              2%

[/]% df -g
            Size            Used           Avail           Used%

            9.8G          199.0M            9.6G              2%

[/]% df -h
usage: df [-k] [-m] [-g] path

   [-k] kilo bytes
   [-m] mega bytes
   [-g] human readable format
   path

Signed-off-by: Zhi Yong Wu <zwu.kernel@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>