Add set/get duplex which sets the frequencies and modes of duplex
VFOs when available.  This required adding Main/Sub VFO emulation
in rigctld, as well as tracking various VFO settings and "emulating"
VFOs.

Now works correctly using an FT-736R in GRig and GPredict.  There
is still some oddness around split operation with the TS-590 though.