Allow link argument to AddrAdd, AddReplace and AddrDel to be nil

[cwmos]

This to address #947

[jellonek]

lgtm

[jellonek]

@aboch, @vishvananda can you take a look? imo this change is clean and simple.

[aboch]

Thank you @cwmos and @jellonek
Would it be possible to add a UT function for this?
Programming and then reading from kernel an address where the link parameter is nil, if not there yet.

[jellonek]

imo it is in the same bucket as checking if user provided non nil addr. we are not checking that what leads to panic, because developer calling this function did a horrible mistake.
UT would be nice to have, but imo is not needed. (i would merge that on your place :P )

[aboch]

If you do not want to add the ut to cover the following two lines

	if link == nil {
		msg.Index = uint32(addr.LinkIndex)

then I have to do it myself.
Believe me, it is going to take longer than one of you guys doing it.
Please add it.

[cwmos]

I agree a unit test is in place here - I don't want this use case to break in the future :-)

I will try to add one.

[aboch]

Thank you @cwmos
Please squash your two commits into one.
This project follows a "one commit per feature/fix/enhancement" approach

[cwmos]

Done @aboch

[aboch]

LGTM