Added DHCPINFORM response for testing WPAD.

[nmoinvaz]

With this PR, I am able to get the WPAD using DHCPINFORM.

Here is are my client and server configs if anybody finds them useful.

var s = dhcpd.createServer({
  // System settings
  range: [
    "192.168.3.10", "192.168.3.99"
  ],
  forceOptions: ['hostname', 'wpad'], // Options that need to be sent, even if they were not requested
  // Option settings
  hostname: "kacknup",
  server: '192.168.0.1', // This is us
  wpad: 'http://wpad.com/wpad.dat'
});

var s = dhcp.createClient({
  mac: "11:22:33:44:55:66", 
  features: ['wpad']
});