Update the ni/lsta reference cycle, add extra checks and assertions.
This is to accomodate problems we were seeing based on net80211
behaviour (join1() and (*iv_update_bss)() as well as state changes for
new iv_bss nodes during an active session).
This should hopefully help to stabilise behaviour until the underlying
problems gets properly addressed (for this and all other device drivers).
XXX-NOTE: this contains last minutes cleanup and one function chnage not
tested yet.
MFC after: 3 days