IfAPI: More accessors

Summary:
Add the following accessors needed by infiniband drivers:

• if_getaddrlen()
• if_setbroadcastaddr()
• if_resolvemulti()

With these accessors, and additional changes on the drivers' side, an
amd64 kernel can be compiled with struct ifnet completely hidden.

Reviewed by: melifaro
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D38488