[calamares] Allow multiple instances if -d is given

- Calamares doesn't like to run multiple instances, since they would
  interfere with each other (stealing disks from each other, for instance).
  The single-application code tries to prevent that.
- For -d runs, for developers where presumably they know what they are
  doing, the single-application restriction is annoying: especially if
  you need two instances at once for some kind of visual comparison.

Drop the single-app requirement if -d is given.

src/calamares/main.cpp

@@ -63,7 +63,13 @@ debug_level( QCommandLineParser& parser, QCommandLineOption& levelOption )
     }
 }
 
-static void
+/** @brief Handles the command-line arguments
+ *
+ * Sets up internals for Calamares based on command-line arguments like `-D`,
+ * `-d`, etc. Returns @c true if this is a *debug* run, i.e. if the `-d`
+ * command-line flag is given, @c false otherwise.
+ */
+static bool
 handle_args( CalamaresApplication& a )
 {
     QCommandLineOption debugOption( QStringList { "d", "debug" },
@@ -100,8 +106,8 @@ handle_args( CalamaresApplication& a )
         CalamaresUtils::setXdgDirs();
     }
     CalamaresUtils::setAllowLocalTranslation( parser.isSet( debugOption ) || parser.isSet( debugTxOption ) );
-    Calamares::Settings::init( parser.isSet( debugOption ) );
-    a.init();
+
+    return parser.isSet( debugOption );
 }
 
 int
@@ -129,13 +135,13 @@ main( int argc, char* argv[] )
     // TODO: umount anything in /tmp/calamares-... as an emergency save function
 #endif
 
-    KDSingleApplicationGuard guard( KDSingleApplicationGuard::AutoKillOtherInstances );
-    if ( guard.isPrimaryInstance() )
+    bool is_debug = handle_args( a );
+
+    if ( !is_debug )
     {
-        handle_args( a );
-        return a.exec();
-    }
-    else
+        KDSingleApplicationGuard guard( KDSingleApplicationGuard::AutoKillOtherInstances );
+
+        if ( !guard.isPrimaryInstance() )
         {
             // Here we have not yet set-up the logger system, so qDebug() is ok
             auto instancelist = guard.instances();
@@ -151,3 +157,8 @@ main( int argc, char* argv[] )
             return 69;  // EX_UNAVAILABLE on FreeBSD
         }
     }
+
+    Calamares::Settings::init( is_debug );
+    a.init();
+    return a.exec();
+}