Introduced by 3b3813fad09eb694a508789394679b6d3378d472.

It wasn't consistent with the naming of other option. The option is
phrased positively to match the Information Model (babel-mac-verify).

Thanks to Julien Muchembled for finding a bug in a previous version of
this commit.

Nodes with default mac-verify would not accept packets from nodes with
non-default mac-verify.
Co-authored-by: Julien Muchembled <jm@jmuchemb.eu>
Co-authored-by: Antonin Décimo <antonin.decimo@gmail.com>

Don’t call `start_message()` since it may cause an infinite loop.

The exact narration is implemented, with all the specified
optimisations.

- neighbour creation is delayed;
- if multiple PCs are found, only the first one is processed, the
  remaining ones are be silently ignored;
- a node ignores all challenge requests contained in a packet except
  the last one;
- and it ignores a challenge request in the case where it is contained
  in a packet with an Index that matches the one in the Neighbour Table
  and a PC that is smaller or equal to the one contained in the
  Neighbour Table.

s/crypto seqno/pc/g
s/challenge response/challenge reply/g
s/challenge req/challenge request/g

Macros starting with __ are reserved for the implementation, it should
not appear in code.

We used to accept packets signed by any key.

This was completely wrong.

Also rename DIGEST_LEN to MAX_DIGEST_LEN.

Flag to allow empty or wrong hmac in the packet trailer.

Known issues:

  - we create a neighbour entry before the first successful challenge;
  - we compute HMAC for each HMAC TLV rather than just once;
  - we only support sending one HMAC TLV;
  - we don't support key rotation.
Co-authored-by: Clara Do <clarado_perso@yahoo.fr>
Co-authored-by: Weronika Kolodziejak <weronika.kolodziejak@gmail.com>