inetutils: several improvement

1. integrate rexec, rlogin, rsh, telnet, tftpd xinetd startup scripts
2. inherit update-alternatives bbclass, not call update-alternatives directly.
3. package all commands into subpackages

Signed-off-by: Roy.Li <rongqing.li@windriver.com>
Signed-off-by: Joe MacDonald <joe.macdonald@windriver.com>