Genie:  Simplify ResolvePathAt()

The same exception is thrown from resolve_pathname() if the cwd argument
isn't a directory.
Genie:  Return vfs::node_ptr from ResolvePathAt()
Genie:  Reduce ResolvePathAt.hh include footprint
Genie -> relix:  Move/rename resolve_path_at()
Merge branch 'resolve_path_at'