Convert netinet6/ to use new routing API.

* Remove &ifpp from ip6_output() in favor of ri->ri_nh_info
* Provide different wrappers to in6_selectsrc:
  Currently it is used by 2 differenct type of customers:
  - socket-based one, which all are unsure about provided
   address scope and
  - in-kernel ones (ND code mostly), which don't have
    any sockets, options, crededentials, etc.
  So, we provide two different wrappers to in6_selectsrc()
  returning select source.
* Make different versions of selectroute():
  Currenly selectroute() is used in two scenarios:
  - SAS, via in6_selecsrc() -> in6_selectif() -> selectroute()
  - output, via in6_output -> wrapper -> selectroute()
  Provide different versions for each customer:
  - fib6_lookup_nh_basic()-based in6_selectif() which is
    capable of returning interface only, without MTU/NHOP/L2
    calculations
  - full-blown fib6_selectroute() with cached route/multipath/
    MTU/L2
* Stop using routing table for link-local address lookups
* Add in6_ifawithifp_lla() to make for-us check faster for link-local
* Add in6_splitscope / in6_setllascope for faster embed/deembed scopes