netinet: rename in_broadcast() to in_ifnet_broadcast()

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.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D49041