Reworked lfsr_mtree_pathlookup a bit to better leverage internal errors

This avoids implicit info, mid.mid=-1 implying a bad path, and mid.mid=0
implying the root directory, at a tradeoff of potentially making the
returned error codes a bit confusing (0 means the file is NOT found!).

Here are the now possible return codes, aside from lower-level errors
(IO, CORRUPT, etc):

- 0      => path is valid, file NOT found
- EXIST  => path is valid, file found
- INVAL  => path is valid, but points to root
- NOENT  => path is NOT valid, intermediate dir missing
- NOTDIR => path is NOT valid, intermediate dir is not a dir

Since the root has no real mdir entry, I think the special INVAL return
code is warranted. It needs special behavior in relevant functions
anyways.

Note that orphaned files still need special handling.

Code changes:

            code          stack
  before:  33944           2944
  after:   34036 (+0.3%)   2944 (+0.0%)

tests/test_dirs.toml

@@ -281,14 +281,14 @@ code = '''
     }
 
     // try to make root, which doesn't make sense
-    lfsr_mkdir(&lfs, "/") => LFS_ERR_EXIST;
+    lfsr_mkdir(&lfs, "/") => LFS_ERR_INVAL;
 
     // make a directory
     err = lfsr_mkdir(&lfs, "ardvark");
     assert(!err || (TEST_PLS && err == LFS_ERR_EXIST));
 
     // try to make root, which doesn't make sense
-    lfsr_mkdir(&lfs, "/") => LFS_ERR_EXIST;
+    lfsr_mkdir(&lfs, "/") => LFS_ERR_INVAL;
 
     // remount?
     if (REMOUNT) {