busybox

Cleanup headers and replace the zero padding with an even smaller version.
One of the pads turned out to be unnecessary: sizeof(struct TarHeader) is
TAR_BLOCK_SIZE, the padding's in the struct.  The others could be done inline
with bb_common_bufsiz1.

This is a cleanup I did to Denis' patch long ago, but got sidetracked by
what turned into svn 15660.