df: more compatible -P behavior

df: more compatible -P behavior

In coreutils df, one of the effects of the option -P is to output
everything in one line. This makes it much easier for a script to parse
the output of df.
This patch adds the same behavior to busybox df.

function                                             old     new   delta
df_main                                              853     863     +10

Signed-off-by: Ralf Friedl <Ralf.Friedl@online.de>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>