sh: Apply set -u to variables in arithmetic.

Note that this only applies to variables that are actually used.
Things like (0 && unsetvar) do not cause an error.

Exp-run done by: pav (with some other sh(1) changes)

bin/sh/arith_yacc.c

@@ -97,6 +97,8 @@ static arith_t arith_lookupvarint(char *varname)
 	arith_t result;
 
 	str = lookupvar(varname);
+	if (uflag && str == NULL)
+		yyerror("variable not set");
 	if (str == NULL || *str == '\0')
 		str = "0";
 	errno = 0;

tools/regression/bin/sh/expansion/set-u3.0

@@ -0,0 +1,6 @@
+# $FreeBSD$
+
+set -u
+unset x
+v=$( (eval ': $((x))') 2>&1 >/dev/null)
+[ $? -ne 0 ] && [ -n "$v" ]