IfAPI: Add if_maddr_empty() to check for any maddrs

if_llmaddr_count() only counts link-level multicast addresses.
hv_netvsc(4) needs to know if there are any multicast addresses. Since
hv_netvsc(4) is the only instance where this would be used, make it a
simple boolean. If others need a if_maddr_count(), that can be added in
the future.

Reviewed by: melifaro
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D39493