I see this commit:

  Change an if_qflush() to an IFQ_PURGE() to deal with ALTQ correctly.

and I find myself wondering `why does if_qflush() even continue to
exist when it won't work with ALTQ'?

But I've also been wondering why IFQ_PURGE() is inlined at all.  It's
not as if it ever occurs in a critical path.  Shouldn't the ALTQ
support just go in if_qflush(), and all the users do a function call
instead?

Is there any high-level design here, or is this all being done seat of
the pants?