Explicitly use "du -k" to set block size in t/test.sh.

Commit e3514812 (Explicitly select the "du" block size in t/test.sh)
used the parameter "-B 1024" which isn't portable. Now use "du -k"
to explicitly get output in kilobytes.

At least du(1) on Linux (GNU coreutils), Mac OS X, FreeBSD, NetBSD,
and OpenBSD support this option.

Signed-off-by: Alexander Barton <alex@barton.de>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
Print a more meaningful error message when help can't run man.

Signed-off-by: Zoran Zaric <zz@zoranzaric.de>
[rlb@defaultvalue.org: elaborated on the error message a bit]
Signed-off-by: Rob Browning <rlb@defaultvalue.org>