[FS#4261] [odhcp6c] Add an option to discard stateless advertisements

[OpenWrt Bugs]

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User who did this - viktor-iarmola (viktor-iarmola) 

Attached to Project - OpenWrt/LEDE Project
Summary - [odhcp6c] Add an option to discard stateless advertisements
Task Type - Bug Report
Category - Base system
Status - Unconfirmed
Assigned To - 
Operating System - All
Severity - Low
Priority - Very Low
Reported Version - Trunk
Due in Version - Undecided
Due Date - Undecided
Details - When odhcp6c runs in `unknown` mode (`-N try` as default and `-P x` without `-F`) it tries to determine its actual mode from received advertisements. 

Normally, when there are both stateful (with IA_NA or IA_PD) and stateless (without both IA_NA and IA_PD) advertisements in the network - it works fine because odhcp6c prefers stateful ones and prioritizes them higher. However, some setups in the wild interfere with this scenario via either of:
  * stateful and stateless advertisements come with the same server DUID - in that case if stateless advertisement comes last we will go into stateless mode despite there being stateful advertisements available;
  * stateful advertisements are "late" and `odhcp6c` only receives stateless ones during its first solicit session.

Both of these cases could seemingly be avoided by `-N force` and/or `-F` options, which will force `odhcp6c` to run in "stateful" mode and ignore advertisements not carrying specific option. However, it is currently impossible to express the intention of wishing to accept only advertisements carrying //either// IA_NA or IA_PD or both of them - and ignoring advertisement without both IA_NA and IA_PD.

To solve this a pull request was created to support a new option for `odhcp6c` - `-D` to discard stateless advertisements. 
Please take a look at the github PR with some extra details: https://github.com/openwrt/odhcp6c/pull/69

More information can be found at the following URL:
https://bugs.openwrt.org/index.php?do=details&task_id=4261

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