Let the link-add functions return the internal pointer to the platform
link object. Similar to link-get, which doesn't copy the link either.

Also adjust the sole users of the add-functions (create-and-realize)
to take the pointer.

Eventually we still copy the returned data, because accessing platform can
invalidate the returned pointer. Thus we don't actually safe any copying
by this (at least every use of the function currently leads to the data
being copied).
Still change it, because I think the API of NMPlatform should look like that.