Fix current and split VFO frequency handling for Kenwood TM-D710(G)

[mikaelnousiainen]

This fixes handling of current and split VFO frequencies on Kenwood TM-D710(G).

The code uses CTRL VFO in the rig for denoting current VFO and PTT VFO for split VFO.

\set_split_freq command now always sets the frequency of the TX VFO, which can be changed using \set_split_vfo command. This works now the way most other rigs handle split too.

To enable split, one can simply do \set_split_vfo 1 VFOB (when current VFO is VFOA). I've tested opposite use of VFOs too (current = VFOB, TX = VFOA).

[mikaelnousiainen]

This fixes #1082

[mikaelnousiainen]

@RhinoLance Please test if this code works for you in split / gpredict use