IfAPI: Add if_altq_is_enabled() interface.

Summary:
The only user of the ALTQ_IS_ENABLED() in a driver checks against the
ifnet queue. Abstract that all out and present the interface to check
if ALTQ is enabled on the interface.

Sponsored by: Juniper Networks, Inc.
Reviewed By: glebius
Differential Revision: https://reviews.freebsd.org/D38204