Tweaked thread-safe implementation

- Stayed on non-system include for lfs_util.h for now
- Named internal functions "lfs_functionraw"
- Merged lfs_fs_traverseraw
- Added LFS_LOCK/UNLOCK macros
- Changed LFS_THREADSAFE from 1/0 to defined/undefined to
  match LFS_READONLY

lfs_util.h

@@ -43,11 +43,6 @@ extern "C"
 {
 #endif
 
-// Enables thread-safe wrappers using the lock/unlock callbacks in lfs_config
-#ifndef LFS_THREADSAFE
-#define LFS_THREADSAFE 0
-#endif
-
 // Macros, may be replaced by system specific wrappers. Arguments to these
 // macros must not have side-effects as the macros can be removed for a smaller
 // code footprint