Add symlink() and readlink()
* Correctly write-lock root on fskit_entry_resolve_root()
* Initializer for symlinks
* Make all but the fskit_entry_set API public (extern "C")
* Add public API method for finding children by name in a directory.
* Expand public path API (extern "C")
* Don't forget to unlock cur_ent and prev_ent when stopping path
resolution on certain errors.
Sort out public API, which has C linkage.  In particular, make methods
that call user callbacks public, so a filesystem can invoke its own
methods via fskit's route dispatching logic if need be.
Add FUSE readlink() and symlink() bindings for fskit_fuse
Only call the open and close callbacks for regular files in the fskit
demo.