df: add support for more options, add some coreutils 6.10 compat.

df: add support for more options, add some coreutils 6.10 compat.
 by Bernhard Reutner-Fischer

function                                             old     new   delta
df_main                                              664     795    +131
packed_usage                                       24812   24862     +50
make_human_readable_str                              213     262     +49
static.ignored_mounts                                  -       8      +8
static.unit_chars                                      -       7      +7
static.zero_and_units                                  6       -      -6
------------------------------------------------------------------------------
(add/remove: 2/1 grow/shrink: 3/0 up/down: 245/-6)            Total: 239 bytes