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.

util/grub-mkstandalone.c

@@ -305,6 +305,7 @@ main (int argc, char *argv[])
   int i;
 
   grub_util_host_init (&argc, &argv);
+  grub_util_disable_fd_syncs ();
 
   files = xmalloc ((argc + 1) * sizeof (files[0]));