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).

bin/sh/expand.c

@@ -657,7 +657,7 @@ evalvar(char *p, int flag)
 	}
 	varlen = 0;
 	startloc = expdest - stackblock();
-	if (!set && uflag) {
+	if (!set && uflag && *var != '@' && *var != '*') {
 		switch (subtype) {
 		case VSNORMAL:
 		case VSTRIMLEFT:

bin/sh/sh.1

@@ -32,7 +32,7 @@
 .\"	from: @(#)sh.1	8.6 (Berkeley) 5/4/95
 .\" $FreeBSD$
 .\"
-.Dd May 31, 2009
+.Dd October 24, 2009
 .Dt SH 1
 .Os
 .Sh NAME
@@ -296,7 +296,10 @@ sh -T -c "trap 'exit 1' 2 ; some-blocking-program"
 .Ed
 .It Fl u Li nounset
 Write a message to standard error when attempting
-to expand a variable that is not set, and if the
+to expand a variable, a positional parameter or
+the special parameter
+.Va \&!
+that is not set, and if the
 shell is not interactive, exit immediately.
 .It Fl V Li vi
 Enable the built-in

tools/regression/bin/sh/expansion/set-u2.0

@@ -0,0 +1,12 @@
+# $FreeBSD$
+
+set -u
+: $* $@ "$@" "$*"
+set -- x
+: $* $@ "$@" "$*"
+shift $#
+: $* $@ "$@" "$*"
+set -- y
+set --
+: $* $@ "$@" "$*"
+exit 0