new features
1.  round robin multiple target macs available via single lifc.
2.  allow interfaces with mismatched mtu to work together.
i.e. compute device mtu as min of devlink mtus in qcfgrsp.
(ideally, we should handle mismatched interfaces but that will
take some restructuring.)
3.  allow rm of target directories.

buggy target defense:
1.  devices not responding to idents could cause retransmit
storms that never time out.  solution: add srb to identify.
2.  ignore qc responses with brodcast addresses.

blame me:
1.  fmtæ's buffer was too small.
2.  redo unit sequencing.  the unit is the nth item in the list.
3.  defend against martian config query responses in qcfgrsp.
4.  squeeze orphan devices properly in netunbind.