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