Change the behaviour of adduser to match the previous incarnation a little more.

If any of the given groups do not exist complain and let the user try again.
This saves the user from discovering at the end of the process that they've
forgotten to add a group or they've typoed.

Thanks to cmc/dougb for pointing out how bad my sh fu actually is.

Original code by: me
Scary sh rewrite by: dougb

Reviewed by:	dougb

usr.sbin/adduser/adduser.sh

@@ -570,8 +570,32 @@ input_interactive() {
 	get_user
 	get_gecos
 	get_uid
-	get_logingroup
-	get_groups
+
+	# The case where group = user is handled elsewhere, so
+	# validate any other groups the user is invited to.
+	until [ "$_logingroup_ok" = yes ]; do
+		get_logingroup
+		_logingroup_ok=yes
+		if [ -n "$ulogingroup" -a "$username" != "$ulogingroup" ]; then
+			if ! ${PWCMD} show group $ulogingroup > /dev/null 2>&1; then
+				echo "Group $ulogingroup does not exist!"
+				_logingroup_ok=no
+			fi
+		fi
+	done
+	until [ "$_groups_ok" = yes ]; do
+		get_groups
+		_groups_ok=yes
+		for i in $ugroups; do
+			if [ "$username" != "$i" ]; then
+				if ! ${PWCMD} show group $i > /dev/null 2>&1; then
+					echo "Group $i does not exist!"
+					_groups_ok=no
+				fi
+			fi
+		done
+	done
+
 	get_class
 	get_shell
 	get_homedir