c_types: rework group()

Fixes #236
id: remove unused arg
c_types: make get_groups/get_group_list public
id: rework id_print()

Fixes #239
id: remove unnecessary coercion
id: remove dead code/unused imports
id: fix typo
Merge pull request #262 from ebfe/fix-group-foo

id/groups: fix group list handling
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
Fix id and groups for Macs.  Also make every util depend on common
common: remove extra cast
sum: handle multiple file args
sum: print filenames
Merge pull request #269 from ebfe/sum

sum: pass busybox tests
Fix build failure for systems with a non-64-bit time_t.