Adjust TX threshold on every TX underrun.  Some supported cards (8139-based) can retransmit on TX underrun and set TOK in addition to TUND.  Also add a check to prevent overflow of the addressable threshold.
Adjust TX threshold on every TX underrun.  Some supported cards (8139-based)
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

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