if: Update IF_ macros comment
ifq: Update comment in net/ifq_var.h
ifq: Add ifq_{is,set,clr}_started to access/modify altq_started
ifq: Add ifq_get_stage to loacte proper altq_stage data
if: if_start_schedule -> ifq_ifstart_schedule

The argument is changed to ifaltq; this eases further changes to
add multiple tx queues support.
if: Add if_devstart_sched to schedule ifnet.if_start call

Mainly to hide ifnet.if_start_nmsg from drivers, avoid code duplication
and ease upcoming multiple tx queues work.
if: if_start_need_schedule -> ifq_ifstart_need_schedule

Consistent function naming as other related functions