newgrp(1): update newgrp's man page to explain why it does not work by
default.  Discourage its use even further.
newgrp(1): Add warning when running newgrp without SETUID

newgrp is 'disabled' by default due to security concerns.  However,
trying to use it does not explain the problem.
ls(1): Add missing flags to usage()

L and p were missing.