Better BOOTP support

[leshow]

• RFC 951 is not currently supported, if that is desired, changes will probably need to be made to dhcproto
• client class support, would be good to distinguish between BOOTP / DHCP in client classes. Perhaps with member('BOOTP') & member('DHCP')
• Pad BOOTP packets to their minimum size of 300 bytes (only some devices require this)