du: fix -s flag

Fixes #241
Merge pull request #263 from ebfe/du

du: fix -s flag
factor: fix uumain
tail: fix uumain
uutils: add chroot, factor, nl, tail
Merge pull request #266 from ebfe/uutils

uutils: add chroot, factor, nl, tail
sum: handle multiple file args
sum: print filenames
Merge pull request #269 from ebfe/sum

sum: pass busybox tests