* Add llt_clear_entry() callback which is able to do all lle
   cleanup including unlinking/freeing
* Relax locking in lltable_prefix_free_af/lltable_free
* Do not pass @llt to lle free callback: it is always NULL now.
* Unify arptimer/nd6_llinfo_timer: explicitly unlock lle avoiding
   unlock/lock sequinces
* Do not pass unlocked lle to nd6_ns_output(): add nd6_llinfo_get_holdsrc()
   to retrieve preferred source address from lle hold queue and pass it
   instead of lle.
* Finally, make nd6_create() create and return unlocked lle
* Separate defrtr handling code from nd6_free():
   use nd6_check_del_defrtr() to check if we need to keep entry instead of
    performing GC,
   use nd6_check_recalc_defrtr() to perform actual recalc on lle removal.
* Move isRouter handling from nd6_cache_lladdr() to separate
   nd6_check_router()
* Add initial code to maintain lle runtime flags in sync.