df: implement -T option

function                                             old     new   delta
df_main                                              863     998    +135
packed_usage                                       29827   29861     +34
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(add/remove: 0/0 grow/shrink: 2/0 up/down: 169/0)             Total: 169 bytes

Signed-off-by: Ari Sundholm <ari@tuxera.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>