Bug 838183: add support for accounts for real people

 * replaces $global_authorized_keys with $admin_users, which are people with admin rights:
   * ssh keys in root's authorized_keys
   * ssh keys in builder's authorized_keys
   * user accounts on all servers
   * unfettered sudo access
 * There's also $users, which just gets you a user account
 * Allows user accounts to ship with dotfiles; mine used as an example
 * Supports multiple SSH keys per person
 * Includes accounts and SSH keys for all releng + moco admins
   (I have a script to sync this from LDAP, but it won't end up in the repo)
 * Purges un-managed users
 * Archives the homedirs of nonexistent users (script by :kang)
 * Tested on Darwin, too, although we're not currently running servers on Darwin