Adapt r197132 to IPv6 stack:   Tighten input checking in in6p_join_group():    * Don't try to use the source address, when its family is unspecified.    * If we get a join without a source, on an existing inclusive      mode group, this is an error, as it would change the filter mode.
Adapt r197132 to IPv6 stack:
  Tighten input checking in in6p_join_group():
   * Don't try to use the source address, when its family is unspecified.
   * If we get a join without a source, on an existing inclusive
     mode group, this is an error, as it would change the filter mode.

  Fix a problem with the handling of in6_mfilter for new memberships:
   * Do not rely on im6f being NULL; it is explicitly initialized to a
     non-NULL pointer when constructing a membership.
   * Explicitly initialize *im6f to EX mode when the source address
     is unspecified.

  This fixes a problem with in_mfilter slot recycling in the join path.

MFC after:	1 day

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase