Dropped LFSR_TYPE_UNKNOWN

Returning the actual on-disk file type is probably more useful for users
as this gives them more information.

I was originally concerned about collisions with future internal types,
LFS_TYPE_TRAVERSAL, etc, needed for internal opened-list tracking, but
it turns out we can avoid problems by starting internal types at 0x80,
since on-disk file types are only 7-bits.

Code changes:

           code          stack
  before: 33710           2592
  after:  33694 (-0.0%)   2592 (+0.0%)

lfs.h

@@ -114,7 +114,6 @@ enum lfs_error {
 // File types
 enum lfs_type {
     // file types
-    LFS_TYPE_UNKNOWN    = 0,
     LFS_TYPE_REG        = 1,
     LFS_TYPE_DIR        = 2,