Use ftruncate on Unix to create hard disk images quickly

Instead of writing out disk blocks slowly across the entire volume,
just use the ftruncate function to create a file instantly at the
desired size.

Depending on file system, this can either result in identical results
to the old code just faster (eg: ZFS and btrfs with compression
enabled), sparse files (most native Unix file systems without
compression, eg ext4 and UFS), or a full non-sparse file like before
(creating an image on FAT).

src/qt/qt_harddiskdialog.cpp

@@ -20,6 +20,9 @@
 #include "ui_qt_harddiskdialog.h"
 
 extern "C" {
+#ifdef __unix__
+#include <unistd.h>
+#endif
 #include <86box/86box.h>
 #include <86box/hdd.h>
 #include "../disk/minivhd/minivhd.h"
@@ -447,6 +450,7 @@ HarddiskDialog::onCreateNewFile()
     }
 
     // formats 0, 1 and 2
+#ifndef __unix__
     connect(this, &HarddiskDialog::fileProgress, this, [this](int value) { ui->progressBar->setValue(value); QApplication::processEvents(); });
     ui->progressBar->setVisible(true);
     [size, &file, this] {
@@ -469,6 +473,13 @@ HarddiskDialog::onCreateNewFile()
         }
         emit fileProgress(100);
     }();
+#else
+    int ret = ftruncate(file.handle(), (size_t) size);
+
+    if (ret) {
+        QMessageBox::critical(this, tr("Unable to write file"), tr("Make sure the file is being saved to a writable directory."));
+    }
+#endif
 
     QMessageBox::information(this, tr("Disk image created"), tr("Remember to partition and format the newly-created drive."));
     setResult(QDialog::Accepted);