• Rename 'struct igmp_ifinfo' into 'struct igmp_ifsoftc', since it really represents a context.
• Preserve name 'struct igmp_ifinfo' for a new structure, that will be stable API between userland and kernel.
• Make sysctl_igmp_ifinfo() return the new 'struct igmp_ifinfo', instead of old one, which had a bunch of internal kernel structures in it.
• Move all above declarations from in_var.h to igmp_var.h, since they are private to IGMP code.

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