Allow inode cache invalidation in high-level API

Allow inode cache invalidation in high-level API

We re-introduce the functionality of invalidating the caches for an
inode specified by path by adding a new routine
fuse_invalidate_path. This is useful for network-based file systems
which use the high-level API, enabling them to notify the kernel about
external changes.

This is a revival of Miklos Szeredi's original code for the
fuse_invalidate routine.