netinet: rename in_broadcast() to in_ifnet_broadcast()

[glebius]

Summary

This aligns with existing in_ifaddr_broadcast() and aligns with other
simple functions or macros with bare "in_" prefix that operator just on
struct in_addr and nothing else, e.g. in_nullhost(). No functional
change.

[glebius]

This is preparation for D49042. Please review them together.