can retransmit on TX underrun and set TOK in addition to TUND. Also add a
check to prevent overflow of the addressable threshold.
This fixes some reports of rl(4) slowness, believed to be related to ALTQ
before.
PR: kern/61448
Submitted by: Tim Draegen-Gilman <timNOeudaemonSPAMnet> (with changes)
MFC after: 1 week