A number of small QOL changes related to lfs_util.h

- Removed stdlib includes from lfs.h, these should all go through
  lfs_util.h to let users override these definitions if stdlib is
  unavailable on their system.

- Changed the name of the LFS_CONFIG macro to LFS_UTIL to avoid
  confusion with the lfs_config struct. This also hints that LFS_UTIL
  is related to lfs_util.h.

  LFS_UTIL allows the user to override lfs_util.h so they can provide
  their own system-level dependencies such as malloc, tracing, builtins,
  stdint definitions, string.h, and others.

- Moved error code definitions to lfs_util.h. This lets users override
  the error codes to replace them with their own error codes and avoid
  a translation layer in some situations. Note the error codes must
  still be in the range of a negative int.

lfs.c

@@ -5,7 +5,6 @@
  * SPDX-License-Identifier: BSD-3-Clause
  */
 #include "lfs.h"
-#include "lfs_util.h"
 
 
 /// Caching block device operations ///