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

sys/net/if.c

@@ -4905,6 +4905,12 @@ if_getcounter(if_t ifp, ift_counter counter)
 	return (ifp->if_get_counter(ifp, counter));
 }
 
+bool
+if_altq_is_enabled(if_t ifp)
+{
+	return (ALTQ_IS_ENABLED(&ifp->if_snd));
+}
+
 struct vnet *
 if_getvnet(if_t ifp)
 {

sys/net/if_var.h

@@ -646,6 +646,8 @@ struct bpf_if *if_getbpf(if_t ifp);
 uint8_t if_getpcp(if_t ifp);
 void *if_getl2com(if_t ifp);
 struct ifvlantrunk *if_getvlantrunk(if_t ifp);
+bool if_altq_is_enabled(if_t ifp);
+
 void *if_getafdata(if_t ifp, int);
 
 /*