paths: Reject dotdots above root

This changes the behavior of paths that attempt to navigate above root
to return LFS_ERR_INVAL:
- before: lfs_stat("..") => 0
- after:  lfs_stat("..") => LFS_ERR_INVAL

This is a bit of an opinionated change while making other path
resolution tweaks.

---

It's a bit unclear exactly what dotdots above the root should do.

POSIX notes: "As a special case, in the root directory, dot-dot may
refer to the root directory itself.", but the word choice of "may"
implies it is up to the implementation.

Originally I implemented this as a loop-root because that's what my
Linux machine does, but I think this may have been the wrong choice.
Because, well, in what world should "/.." not be an error?

Long-term, this should help be more consistent with openat-like APIs,
where ".." should either error with LFS_ERR_INVAL or actually navigate
above the relative path. "Saturating" the ".." navigation really doesn't
make sense in this case.

lfs.c

@@ -1500,13 +1500,17 @@ static lfs_stag_t lfs_dir_find(lfs_t *lfs, lfs_mdir_t *dir,
         }
         lfs_size_t namelen = strcspn(name, "/");
 
-        // skip '.' and root '..'
-        if ((namelen == 1 && memcmp(name, ".", 1) == 0) ||
-            (namelen == 2 && memcmp(name, "..", 2) == 0)) {
+        // skip '.'
+        if (namelen == 1 && memcmp(name, ".", 1) == 0) {
             name += namelen;
             goto nextname;
         }
 
+        // error on unmatched '..', trying to go above root?
+        if (namelen == 2 && memcmp(name, "..", 2) == 0) {
+            return LFS_ERR_INVAL;
+        }
+
         // skip if matched by '..' in name
         const char *suffix = name + namelen;
         lfs_size_t sufflen;