ip_mroute: Try to make function pointer declarations more consistent

[markj]

Summary

The ip_mroute and ip6_mroute modules hook into the network stack via
several function pointers. Declarations for these pointers are
scattered around several headers. Put them all in the same place,
ip(6)_mroute.h.

No functional change intended.

[glebius]

sys/netinet/ip_mroute.h

362–366

I don't think any modern compiler actually needs this any more.