dhcp: nettools: support option 249 (Microsoft Classless Static Route)

Beniamino Galvani requested to merge bg/dhcp-nettools-option249 into main

From [1]:

  The length and the data format for the Microsoft Classless Static
  Route Option are exactly the same as those specified for the
  Classless Static Route Option in [RFC3442]; the only difference is
  that Option Code 249 should be used instead of or in addition to
  Option Code 121.

Use routes from option 249 when option 121 is not present, as already done by the dhclient backend.

[1] https://docs.microsoft.com/en-us/openspecs/windows_protocols/ms-dhcpe/f9c19c79-1c7f-4746-b555-0c0fc523f3f9


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