Speed up test suite by avoiding fsync

Add grub_util_disable_fd_syncs call to turn grub_util_fd_sync calls into
no-ops, and use it in programs that copy files but do not need to take
special care to sync writes (grub-mknetdir, grub-rescue,
grub-mkstandalone).

On my laptop, this reduces partmap_test's runtime from 1236 seconds to
204 seconds.

grub-core/osdep/aros/hostdisk.c

@@ -455,6 +455,8 @@ grub_util_fd_close (grub_util_fd_t fd)
     }
 }
 
+static int allow_fd_syncs = 1;
+
 static void
 grub_util_fd_sync_volume (grub_util_fd_t fd)
 {
@@ -469,17 +471,26 @@ grub_util_fd_sync_volume (grub_util_fd_t fd)
 void
 grub_util_fd_sync (grub_util_fd_t fd)
 {
-  switch (fd->type)
+  if (allow_fd_syncs)
     {
-    case GRUB_UTIL_FD_FILE:
-      fsync (fd->fd);
-      return;
-    case GRUB_UTIL_FD_DISK:
-      grub_util_fd_sync_volume (fd);
-      return;
+      switch (fd->type)
+	{
+	case GRUB_UTIL_FD_FILE:
+	  fsync (fd->fd);
+	  return;
+	case GRUB_UTIL_FD_DISK:
+	  grub_util_fd_sync_volume (fd);
+	  return;
+	}
     }
 }
 
+void
+grub_util_disable_fd_syncs (void)
+{
+  allow_fd_syncs = 0;
+}
+
 void
 grub_hostdisk_flush_initial_buffer (const char *os_dev __attribute__ ((unused)))
 {

grub-core/osdep/unix/hostdisk.c

@@ -191,10 +191,19 @@ grub_util_fd_strerror (void)
   return strerror (errno);
 }
 
+static int allow_fd_syncs = 1;
+
 void
 grub_util_fd_sync (grub_util_fd_t fd)
 {
-  fsync (fd);
+  if (allow_fd_syncs)
+    fsync (fd);
+}
+
+void
+grub_util_disable_fd_syncs (void)
+{
+  allow_fd_syncs = 0;
 }
 
 void

grub-core/osdep/windows/hostdisk.c

@@ -240,10 +240,19 @@ grub_util_fd_write (grub_util_fd_t fd, const char *buf, size_t len)
   return real_read;
 }
 
+static int allow_fd_syncs = 1;
+
 void
 grub_util_fd_sync (grub_util_fd_t fd)
 {
-  FlushFileBuffers (fd);
+  if (allow_fd_syncs)
+    FlushFileBuffers (fd);
+}
+
+void
+grub_util_disable_fd_syncs (void)
+{
+  allow_fd_syncs = 0;
 }
 
 void