Explicitly give unit values

Removes the ability to hide symbol units in file formats.  This was
mostly the case for a while but there remained corner cases where we
left a default/hidden option

Fixes https://gitlab.com/kicad/code/kicad/issues/10888

eeschema/dialogs/dialog_lib_symbol_properties.cpp

@@ -591,7 +591,7 @@ void DIALOG_LIB_SYMBOL_PROPERTIES::OnEditSpiceModel( wxCommandEvent& event )
 {
 #ifdef KICAD_SPICE
     int diff = m_fields->size();
-    auto symbol = SCH_SYMBOL( *m_libEntry, m_libEntry->GetLibId(), nullptr );
+    auto symbol = SCH_SYMBOL( *m_libEntry, m_libEntry->GetLibId(), nullptr, 0 );
 
     DIALOG_SPICE_MODEL dialog( this, symbol, m_fields );
 

eeschema/sch_plugins/kicad/sch_sexpr_plugin.cpp

@@ -685,14 +685,12 @@ void SCH_SEXPR_PLUGIN::saveSymbol( SCH_SYMBOL* aSymbol, SCH_SHEET_PATH* aSheetPa
         m_out->Print( 0, ")" );
     }
 
-    int unit = -1;
+    int unit = aSymbol->GetUnit();
 
-    if( !( aSymbol->GetInstanceReferences().size() > 1 ) )
-        unit = aSymbol->GetUnit();
-    else if( aSheetPath != nullptr )
+    if( ( aSymbol->GetInstanceReferences().size() > 1 ) && aSheetPath )
         unit = aSymbol->GetUnitSelection( aSheetPath );
-    if ( unit >= 0 )
-        m_out->Print( 0, " (unit %d)", unit );
+
+    m_out->Print( 0, " (unit %d)", unit );
 
     if( aSymbol->GetConvert() == LIB_ITEM::LIB_CONVERT::DEMORGAN )
         m_out->Print( 0, " (convert %d)", aSymbol->GetConvert() );

eeschema/sch_symbol.h

@@ -92,7 +92,7 @@ public:
      * @param setNewItemFlag is used to set the symbol #IS_NEW and #IS_MOVING flags.
      */
     SCH_SYMBOL( const LIB_SYMBOL& aSymbol, const LIB_ID& aLibId, const SCH_SHEET_PATH* aSheet,
-                int unit = 0, int convert = 0, const VECTOR2I& pos = VECTOR2I( 0, 0 ) );
+                int unit, int convert = 0, const VECTOR2I& pos = VECTOR2I( 0, 0 ) );
 
     SCH_SYMBOL( const LIB_SYMBOL& aSymbol, const SCH_SHEET_PATH* aSheet, const PICKED_SYMBOL& aSel,
                 const VECTOR2I& pos = VECTOR2I( 0, 0 ) );