plotter: We we trying to export mm but chose in

Note that the comment in line 142 where the scale factor was set had the
correct value to indicate metric (1) but in the actual boilerplate, we
set imperial (0).  This has been around for 6-8 years, so it would seem
DXF is not our most popular export.

Note, I initially tried to use '1' to specify metric units but this was
not handled by DXF readers (QCAM, Inkscape).  Instead, scaling the page
units to inches properly seems to work.

(cherry picked from commit 0eb391b13f)

common/plotters/DXF_plotter.cpp

@@ -139,14 +139,14 @@ void DXF_PLOTTER::SetViewport( const wxPoint& aOffset, double aIusPerDecimil,
     paperSize.y = 0;
 
     /* Like paper size DXF units are abstract too. Anyway there is a
-     * system variable (MEASUREMENT) which will be set to 1 to indicate
-     * metric units */
+     * system variable (MEASUREMENT) which will be set to 0 to indicate
+     * english units */
     m_IUsPerDecimil = aIusPerDecimil;
     iuPerDeviceUnit = 1.0 / aIusPerDecimil; // Gives a DXF in decimils
-    iuPerDeviceUnit *= 0.00254;             // ... now in mm
+    iuPerDeviceUnit *= 0.0001;              // ... now in inches
 
     SetDefaultLineWidth( 0 );               // No line width on DXF
-    m_plotMirror = false;                     // No mirroring on DXF
+    m_plotMirror = false;                   // No mirroring on DXF
     m_currentColor = COLOR4D::BLACK;
 }
 
@@ -176,7 +176,7 @@ bool DXF_PLOTTER::StartPlot()
            "$MEASUREMENT\n"
            "  70\n"
            "0\n"
-           "  0\n"              // This means 'metric units'
+           "  0\n"              // This means 'english units'
            "ENDSEC\n"
            "  0\n"
            "SECTION\n"