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-mknetdir.c

@@ -171,6 +171,7 @@ main (int argc, char *argv[])
   const char *pkglibdir;
 
   grub_util_host_init (&argc, &argv);
+  grub_util_disable_fd_syncs ();
   rootdir = xstrdup ("/srv/tftp");
   pkglibdir = grub_util_get_pkglibdir ();