mech-v2: unidirectional tunneling removed, SLLA/TLLA changes

[Pekka Savola <pekkas@netcore.fi>]

Hi,

Two status issues on certain mech-v2 features:

1) Based on Chirayu's comments earlier, I've changed the rule (from
SHOULD NOT) to MUST NOT send TLLA/SLLA options on configured tunnels.  
I believe some very old implementations, ages ago, did this, but it
seems to make sense to get them fixed if they want to conform to the
new specification. (See below.)  This does not create a severe interop
problem as long as the other end implements the "MUST ignore" as well.

2) As nobody has offered support for unidirectional tunnels, I removed
them from the mech-v2 revision as well.

If you have any objections, please raise them soon, and provide 
suggested alternatives.

======
   For the purposes of Neighbor Discovery the configured tunnels speci­
   fied in this document are assumed to NOT have a link-layer address,
   even though the link-layer (IPv4) does have address.  This means
   that:
                                                                                    
    -   the sender of Neighbor Discovery packets MUST NOT include Source
        Link Layer Address options or Target Link Layer Address options
        on the tunnel link.
                                                                                    
    -   the receiver MUST, while otherwise processing the neighbor dis­
        covery packet, silently ignore the content of any Source Link
        Layer Address options or Target Link Layer Address options
        received on the tunnel link.