Split in6_selectsrc() into in6_selectsrc_addr() and in6_selectsrc_socket().

Split in6_selectsrc() into in6_selectsrc_addr() and in6_selectsrc_socket().

in6_selectsrc() has 2 class of users: socket-based one (raw/udp/pcb/etc) and
  socket-less (ND code). The main reason for that change is inability to
  specify non-default FIB for callers w/o socket since (internally) inpcb
  is used to determine fib.

As as result, add 2 wrappers for in6_selectsrc() (making in6_selectsrc()
  static):
1) in6_selectsrc_socket() for the former class. Embed scope_ambiguous check
  along with returning hop limit when needed.
2) in6_selectsrc_addr() for the latter case. Add 'fibnum' argument and
  pass IPv6 address  w/ explicitly specified scope as separate argument.

Reviewed by:	ae (previous version)