paths: Reject dotdots above root

This changes the behavior of paths that attempt to navigate above root
to now return LFS_ERR_INVAL:

- before: lfs_stat("../a") => 0
- after:  lfs_stat("../a") => LFS_ERR_INVAL

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

In terms of POSIX-compatibility, 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.

I originally implement this as a root-loop simply because that is what
my Linux machine does, but I now think that's not the best option. Since
we're making other path-related tweaks, we might as well try to adopt
behavior that is, in my opinion, safer and less... weird...

This should also help make paths more consistent with future theoretical
openat-list APIs, where saturating at the current directory is sort of
the least expected behavior.

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;