Fix id and groups for Macs.  Also make every util depend on common
common: remove extra cast
common: make get_group_list work for all users on Macs
common: fix for Linux
Begin using dependency info in the Makefile
id: remove warning about unused import
Merge pull request #268 from Arcterus/id-group-mac-fix

Fix id and groups for Macs.  Also add dependency information.
c_types: remove unused import on linux
Merge pull request #275 from ebfe/c-types-import

c_types: remove unused import
common: import Mac-specific things in one statement
Merge pull request #276 from Arcterus/c_types-mac-import

common: import Mac-specific things in one statement
du: tweak output format
du: exit with status 1 on invalid arguments
Merge pull request #277 from ebfe/du

du: exit with status 1 on invalid arguments