MFC r198454: sh: Exempt $@ and $* from set -u

MFC r198454: sh: Exempt $@ and $* from set -u

This seems more useful and will likely be in the next POSIX standard.

Also document more precisely in the man page what set -u does (note that
$@, $* and $! are the only special parameters that can ever be unset, all
the others are always set, although they may be empty).

git-svn-id: svn://svn.freebsd.org/base/stable/8@215066 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f